Return ticket

I fly out/in of Guatemala 6-8 times a year and a return ticket has never been required. The airlines enforce this not Guate immigrations.

You are definitely wrong in your theories.

Guatemala is in fact one of the least obtrusive countries in this regard and just wants people to come, no matter what. This is STRICTLY the airlines attempting to make extra $; it is their call, and IMO there is zero chance of them getting 'fined' as they flame. There are, OTOH, countries in the region who are more involved in these policies: Costa Rica and Panama come to mind.

I am extremely surprised that American Airlines is doing it as I've flown them one way at least 4 times and never had to produce. Regardless, it's an easy formality to get around, just bring a sample itinerary or a bus ticket.

Agreed, it's just the airlines covering their asses - nothing to do with Guatemala at all.

The law says that if Guatemala turns you back for not having onward proof the airline has to foot the bill to fly you back where you came from.

Last time they pulled me up on it I had a return flight to Guatemala with Aeromexico and they weren't going to let me board. But after about two minutes I think they realized what a useless charade it all was, and made me a reservation on an outgoing flight in a weeks' time and printed that up so that if Guatemalan immigration asked for proof of onward travel I had a piece of paper to wave at them.

The reservation (it was like a pre-reservation, or an intent-to-reserve or some such airline jargon) cost nothing and didn't even need to be cancelled - the conditions were that if I didn't confirm (and pay) within 72 hours, it expired.

Which all worked fine, but I admit it all revolves around actually ending up with a human on the other side of the check in counter.

Airlines need to comply with the rules of Immigration in a given country, even when the country is not enforcing them. This is also true for Costa Rica and Panama. In fact, in Panama, you need proof of Onward Travel at the Borders, or they make you buy a $14 bus ticket returning to Costa Rica. Airlines can be fined.

Also, you can avoid this by flying oneway to Cancun MX and then head down to Belize/Border. MX is no issue flying OW from most countries,

Airlines need to comply with the rules of Immigration in a given country, even when the country is not enforcing them

Nah... they definitely don't 'need' to do this, and in the vast majority of cases they don't even bother. It's pretty much a purely human/random process (and my experiences includes having issues on around 2 out of 30+ one-way flights over the last several years). Not claiming to be an expert on the mechanics of it but I'm absolutely sure that Guatemala is not fining these companies; other countries, from the information that's available, do it in some rare cases. It's not the case here and they are desperate for more tourists arriving by air.

Mexico is pretty much identical in this regard: they have the same technical requirements on the books, TT posters have reported a couple of problems with airlines on departure even though it's fine 95% of the time, etc. There is zero advantage whatsoever of changing travel plans to there for this reason.

Not so #5.....Solohobo is correct....the boarding airlines have an absolute responsibility to ensure their passengers comply with the set entry requirements including onward travel before allowing them to board. Fines are unlikely in practice only because immigration rarely check for onward travel (but they will if they are looking for a reason to deny entry). Whilst it has potential for the airlines to make money selling additional tickets the whole thing is a hassle that many check-in staff still can't be bothered with, but it looks like its being tightened up gradually and I wouldn't be surprised if its cast in stone pretty soon (including the types of "proof" that are acceptable)....
Watch This Space....

I was driving from Canada to Costa Rica once and they wouldn't let me enter the USA because I didn't have an "onward ticket". I said "what sort of onward ticket could I have if I'm driving a car?"....they said "but how did you get here without an onward ticket?"...I said "I flew into Canada on a one-way"....they said "you should never have been allowed to board the aircraft". Nice...I (Heart) Merkins....

I agree that this is being tightened up and becoming more prevalent. In the past it seemed to be only flights originating in or entering the U.S. and then usually just Spirit but now it is universal with all airlines to enter Costa Rica. When you present a proof of residency card which waives you from this requirement the person at the check in desk always disappears with the card momentarily to do what? Authenticate this credential? Ask a supervisor if it is O.K.? Copy it?
Recently I flew one way from Bogota to San Jose on Copa and I went through this whole scene at El Dorado. Definitely here to stay.
There are online businesses that will buy and deliver a Ticabus or Transnica ticket out of Costa Rica, at a very inflated price I would add, that satisfies the law even if you do not intend to actually use it. The whole thing in general has the contrived appearance of a money grubbing enterprise in my opinion.
The only way to truly circumvent these laws is to become a legal resident of the country you intend live in. As immigration and its implications becomes more of a concern, even in developing countries, the air carriers will have no choice but to enforce these policies. Guatemala included.

It's the airlines obligation to make sure you have all the documents required by the country where your destination is before letting you board the flight. They should be checking you for visa, onwards ticket, etc.I believe they use the IATA database, you can have a look what the requirements are - http://www.iatatravelcentre.com/travelinformation.php

The airlines (TACA, COPA) have always checked if I am holding an onward ticket out of Guatemala in less then 3 months... I have never been asked for such thing by Guatemala immigrations though, so it's sort of stupid to require such a ticket as there are other ways to leave GT in 3 months without flying...

I think any per-purchased ticket with valid PNR/itinerary will do the job... And it can be cancelled once here... All this does not make much practical sense apart of the airlines trying to sell you under pressure an extra ticket I assume...