The airline does not care if you buy 8 quarter tickets RT or a 4 part one way if they sold them this way. It is the particular country you are flying into that cares how long you plan to be in their country. Some countries are more strict than others, Cuba is the only one where I have been escorted to buy a ticket out of country when I have landed on a one-way ticket.
Cuba use to only charge $25 for a change in flight, so it's not worth it IMO.
The same thing would happen to you in Costa Rica if you entered with a one-way ticket Themis. And yes the airline does care because if you are not allowed to enter, they have to take you back from whence you came, and if there might be no spare seats available on the plane.
The airlines care if there are such written and agreed on regulations; otherwise, they do not because they will not get fined or risk losing landing rights. For many reasons, Cuba rejects entry to people all the time (compared to other countries) and they are back on the next plane to where they came from. I know of no one that was denied entry because they did not have a return ticket. The worst that happens is you are forced to buy one and then allowed in.
