The situation with Taca from SFO is you are not flying USA to Belize you are Flying USA to El Salvador, different rules when flying El Salvador to Belize as this case is. However I know the majority of Tacas flights go to El Salvador first which changes immigrations rules I expect not all flights do. So if you are on a direct flight on TACA from US to Belize they may also want the RT ticket.

Oh and yes it is rules set by but not inforced by immigration, it is inforced by airlines at departure.

When I check in for flights from the US to go back home to Belize I usually travel on my UK passport. The airline ticket desk always asks why I don't have a return ticket (to leave Belize) and I have to provide proof that I can legally reside in Belize before they will check me in.

So, the airlines do enforce the immigration protocol - chances are most folks don't realize it because the ticket counter knows whether or not your have a return ticket.