Airlines are also responsible for the cost/inconvenience of taking anyone they carry to the US - who is then refused entry at their port of arrival - back to where they carried them from. So the airlines are strongly motivated to do the government's bidding ... which is of course the way the screening legislation was designed to function.
You read 'round trip' and think it means 'round trip'. In fact what the rule is saying is you must show proof you intend to leave the USA. That MAY be a round trip ticket. It MAY be an onward ticket (not the same thing is it). It MAY be any other explanation or 'proof' that the Immigration officer chooses to accept.
Let's take an example. You are a recreational sailor. You fly from Europe to San Diego with the intent of joining a yacht that is sailing to Hawaiii and points west. What happens to the apparent need for a return ticket? Obviously, you are not going to be returning to San Diego or from Hawaii necessarily to Europe. You could be going to sail on to the French Polynesians or Australia or Japan or India or anywhere. So what will they say when you arrive in San Diego?
Suppose you have a letter from the owner/skipper of the yacht you intend to join, inviting you to crew on the boat. Will Immigration accept that as 'proof' that you intend to leave the USA? Quite likely if you have a duffel bag with obvious sailing gear in it. Suppose you don't have a letter but you do have a cellphone number for the owner/skipper that Immigration can call and ask if your story is true? Quite likely that will get you through.
Here is the only part of the quote you gave bjookaj that matters, "will transport the traveler out of the United States to any other foreign port or place" ANYTHING that will satisfy Immigration that that is what you intend to do will get you through.
Why the airlines insist on it is clear. It costs them money if you are turned back. But it is not Immigration that insists on a return ticket. They will accept any reasonable alternative.
The rule is written and applies to probably 95%+ of all travellers. They all return to where they came from. ie. home. But that does not mean there are not exceptions to the rule that are allowed. They are allowed every day.
Of course there are exceptions, and discretion and judgement are applied.
But the salient reality for this branch is, if I went to Melbourne Airport this morning (just up the road) with a valid passport, a valid ESTA, but with just a one-way ticket to LAX and no visa, Qantas would not led me board ... I think that is close to a 100% certainty - despite any pleadings I made about going sailing, or motoring to Costa Rica, or whatever. And the example of the sailor is not a particularly good exception - visa classes do exist for people who live or work on boats.

You are a recreational sailor. You fly from Europe to San Diego with the intent of joining a yacht that is sailing to Hawaiii and points west. What happens to the apparent need for a return ticket?
False analogy.
Recreational sailors are required to have a visa, because they are using private transport. Visa holders are not required to have a ticket, as this is a provision of the VWP only.
The two instances are not comparable.
Suppose you have a letter from the owner/skipper of the yacht you intend to join, inviting you to crew on the boat.
Again, false analogy.
You need a (D-)visa to crew the boat.
Here is the only part of the quote you gave bjookaj that matters, "will transport the traveler out of the United States to any other foreign port or place"
Now you're just taking things out of context.
Let me add the context back:
>and at the time of arrival must have a round trip ticket that will transport the traveler out of the United States to any other foreign port or place
And if you refer to me, I'd appreciate it if you'd spell my handle correctly.
Why the airlines insist on it is clear.
Yes, it is--because they are required by rule to check.
But it is not Immigration that insists on a return ticket.
I quoted the rule, which states otherwise.
You're welcome to believe what you wish, as wrong as it is.
I had no intention to mis-spell your name bzookaj. Just a typo.
I'm not going to get into a whole discussion re yacht crew and visas. There is the law and there is common practice. They differ considerably with everyone involved knowing they do. It depends on whether the boat is going to spend any time in a country. A private yacht from Canada stops in San Diego to provision. Technically, all crew need a B1 visa (or a D visa for paid commercial ship crew). But all they are doing is provisioning and moving on to Mexico, (then Hawaii, then Tahiti, then Japan, then Indonesia, then India, then S. Africa, then Australia, etc, etc.) . Immigration will not deny them being allowed to provision. They may insist some crew members not get off the boat or they may not depending on nationality. If a new crew member joins from another country they may insist that crew member not leave the boat. They may insist the owner/skipper hire someone to do the provisioning and bring the provisions to the boat and no one leave the boat. They may allow the owner/skipper and crew ashore but insist they sail on the same day or in X days. It is not a simple black and white issue. Crew and visas is a very grey area. Some owners/skippers could insist all crew have visas for all countries they intend to or even just might visit. Imagine the nightmare that would involve. Often the answer is simply, you don't get off the boat in harbour if Immigration tells you you can't. People who live aboard yachts full time and sail the world do not always have a visa for every country they visit. Crew join and leave boats all over the world. Someone joins for a month then leaves and someone else takes their place. They don't all have visas. It's simply not practical. They enter as a tourist and then leave with no one the wiser.
But let's not get off track on boat crews. Let's stick to the 'return ticket' issue. One issue at a time. ;-)
All I intended was an example of a situation in which a return or onward ticket made no sense. You can think of any scenario you wish to provide an example of that and what would happen as a result. Here is another (hopefully less controversial) example.
A traveller is flying to New York with the intention of travelling (as a tourist) across the US to Los Angeles before making his way north to Vancouver, Canada. How long it will take is undetermined as he has no set itinerary at all. But he does know he must leave the US in 90 days. He intends to travel by train and bus. He will continue on from Canada to somewhere else in the world. He intends to travel for 2 years and has no fixed itinerary but has $50,000 cash in a bank account to fund his travel.
Under no circumstances would he have a return ticket. If you want, you could assume he has a Seattle to Vancouver bus ticket as an indication of onward travel. Or he doesn't and intends to state his plans to Immigration on arrival and show he has sufficient funds (online bank balance) for his time in the US and to fund leaving the US.
Immigration will not insist on a return ticket. It makes no sense. The airline may insist ianw in which case you buy a one way, fully refundable ticket from Quantas, New York to Melbourne, right then, right there at their desk in Melbourne and on arrival in NYC you return the ticket for a full refund at their desk before leaving the airport. You cannot stop an airline from following an inflexible policy but you can easily circumvent it. They do not insist because US Immigration insists, the airline insists because the airline is covering their butt.
What makes that clear is that many airlines insist on return tickets even when the country you are flying to clearly does not. For example, Quantas will no doubt insist you have a return ticket if you fly to Canada from Melbourne but Canadian Immigration regulations CLEARLY do not insist that you have a return ticket to enter Canada. Their regulation specifically does not mention it but does say you 'must satisfy Immigration you intend to leave'. I live in Canada and know that some airlines insist on return tickets while others do not. The same is true of the UK. Immigration there also uses the 'satisfy' word in their regulation and airlines interpret it as 'return ticket'.
The airline will argue that means you have to have a return ticket. They base the argument on the supposed 'proof' that constitutes. It proves nothing but it covers their butt if you are refused entry. They don't get fined and they have been paid already to fly you back to Melbourne.
All I am saying is that any regulation of this kind is not 100% enforced. Discretion and common sense can be applied by the individual Immigration officer. What does apply 100% of the time is that you must SATISFY the Immigration officer that you do not intend to work and you do intend to leave the country within the 90 days.
What an airline does is a totally separate issue from that and their reasons for doing so are NOT because the Immigration of a country makes them do it.

Travelinstyle is actually absolutely correct here. When the law writes "round trip ticket" that doesn't have to be a physical ticket. As long as you can satisfy the officer that you'll leave the US within 90 days, you're good. This has been shown over and over again, and has been verified by direct question to CBP and even the internal CBP rulebook dictates that a return/onward ticket is not required and normally not something asked for.
Why is it so hard to understand? Most countries in the world have official rules that differ from the actual rules. Take Russia for example, for which I am in the process of applying for a visa. According to official rules, you need to have paid accommodation for your entire stay in Russia, which in practice would make it impossible to backpack through the country. However, in practice, this is not something that is required. Similarly, the government of Iraq claims that it is impossible and illegal to enter Iraq from Turkey without visa. But it's not. It's trivial, and border guards let you in smiling, I've done it myself. These discrepancies, and many more are widely discussed on this forum, and travelers use them successfully all the time. USA is no different than Russia and Iraq in this regard. The rules say one thing, what happens in practice is something else.
I am not in any way advocating breaking the law. I'm just telling what the situation is actually like. Travelers are allowed to do whatever they like, but they should know that Travelinstyle and I are likely the most experienced travelers in this thread when it comes to international travel (in terms of crossing various borders and dealing with immigration), and are the ones most likely to know how things work in practice, and we are telling you that a return/onward ticket is not a requirement in practice with US immigration.
For reference, here is what CBP says when directly asked a question that is almost identical to that which Travelinstyle describes in #34:
Please be advised A round trip ticket is not required to apply for ESTA or apply for entry, however upon entry you will need to provide proof of residency, solvency, and intent to return.
LOL, actually, 'proof of residency and intent to return' can also get a lot of people all worked up. The guy leaving home for 2 years has given up his home in X when he left and may or may not intend to ever return, who knows.
But he can show solvency and he can explain his plan and he can say he knows he has to leave THIS country (whichever it is he is entering) within the allowed period of time.
It ALL comes down to SATISFYING Immigration of your INTENTIONS. In reality, it's all about perception. If Immigration perceives you as having intentions they do not approve of (likely to try and work illegally or overstay) they will deny you entry with or without return or onward tickets (they prove nothing about your intentions and will be ignored).
What do you think most illegal immigrants who enter by air do? They all have return tickets and Immigration KNOWS that.

What do you think most illegal immigrants who enter by air do?
Your fallacies would be humorous if they weren't so misleading and egregious.
They do not break that rule. They break the rule regarding length of stay.
This is simple:
You stated that it was not an immigration rule, but an airline rule. It's not, and you are wrong on that account. I have quoted the acutal rules to show such.
If you don't like them, please take it up with the rulemakers.
Until then "the rules are what they are."
And please do not advocate ignoring them.
Bjookaj, what is your problem. People arrive by air with return tickets who do not intend to use them. My point is and was that a return ticket does not constitute 'proof' of anything. Immigration know that and as I have said, ignore the return ticket if they think the person intends to stay and work illegally.
Yes, they don't break the 'rule' of having a return ticket. So what? All that indicates is that the rule is meaningless in terms of actually stopping anyone or being accepted by Immigration as proving anything.
No, I did NOT state it was not an immigration rule. I stated that it was not a 100% enforced rule. That is not the same thing as saying the rule did not exist. So I am NOT WRONG on that account.
This isn't like 'thou shall not kill'. It isn't black and white when it comes to enforcement. Immigration is flexible in how they interpret and apply the regulations.
Airlines, have policies and often are far less flexible. Airlines and Immigration are two different entities. EACH does what each does.
I am not advocating ignoring the regulations, I am advocating knowing what they really mean and how they are really applied. The 'rules are what they are' yes, but you just don't seem to understand (or are being deliberately obtuse) that they are not enforced 100%. Exceptions are made every day. They have to be based on common sense.
Let me ask you this simple question bzookaj. What nationality are you and what experience do you have of trying to enter the USA without a return ticket?