All airlines may not have the same policies. Better to check first. Or, if they refuse to let you board without a return, just buy a fully refundable ticket at the counter and immediately refund it when you land.
Same for us we were refused boarding with out a return ticket. Bought one and as soon as we landed turned it in. Only hangup was it took almost a month before the actual refund hit our account.
For only about $12 I bought a 'through ticket" and then turned in. Once I did this I could start buying round trip tickets originating here. Never had a problem since.
As of July 2013 United one way flight into Belize was not a problem at all. Neither airline or immigration questioned it. If others have had issues then it may be due to something other than "Immigration" policy.
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