First, most countries usually have some sort of regulation/rule/law that any visitor must have an onward ticket out of the country they are entering. Without an onward ticket, you COULD be refused entry. It helps if you carry proof that you have sufficient funds to support yourself, and your appearance is such that you appear to have money to support yourself.
I've been around the world a few times, and only once on a 6 week visit to the UK was I ever asked about a return ticket. I've never been asked for one when entering Belize, or entering Mexico on my way to Belize.
The issue with the airlines, as I understand it, is that the airline has to pay the cost to get you back to your departure point IF you are refused entry. Therefore, it's the airline (and/or the travel agent) that tries to ensure you have a return ticket.
In Belize, the initial 30-day stamp can be easily renewed at any immigration office. This is one common way of beginning the process to obtain permanent residency, so (and I don't know if anyone has tried this) if anyone questions you, just say you are planning to get residency, and you have to stay in the country for 12 months continuously. I'm sure the Belizean officials understand this. After all, for the first 12 months, they still have the option every month of not renewing the 30 days.