Earlier quoted context omitted.
It's the classic I've seen used in jokes and comic strips - entry is free; once you enter, you discover that leaving costs money.
That's normal in the service industry, or in shops, for example. You get what you want, then pay on leaving. If you don't pay they detain you. Try leaving your local restaurant without someone paying for the food. They won't care if you paid your friend for the meal and he kept the money.
Incidentally, lower-end restaurants tend to be "pay before you eat", just like fast food joints. I honestly prefer it that way; saves the cognitive burden and a lot of time (sometimes waiting for a waiter to come with the receipt can take as much time as it took to eat the food).