Don't understand don't allow air or boat connections. Once you land can't you make your own connections.
I am with you, I didn't understand that either, but I think it has to do with accessibility in case a flight has been delayed till the next day. These are charter airlines, that means that they are not on a tight schedule like traditional airlines and also touroperators, that means that they want book your hotel as well. When they have to move back a plane load of tourists to the hotel zone, air and water transfers are to expensive and time consuming. Also when they have to sleep the tourists in a different area (like Belize City) they have to deal with hotels they don't have contracts with, thus they have to pay top dollar when there are enough accommodations, which they likely don't have.