Personally, I think that were I hosting such an event, I wouldn't make the choices so open if I didn't have enough money to comfortabley pay for anything my guests might do. And if I had enough money to do that, I would think that I would actually be happy if my guests really enjoyed themselves by getting a treat that they ordinarily wouldn't get for themselves.
The exception to this would be, I think, if a guest who has never actually had, for example, lobster,and ordinarily would get chicken, decides to try the lobster because it is on my dime and then makes a big deal over how gross it is and throws it out and demands I pay a second time for something else. Or if a guest made a big deal out of the thing, loudly saying to everybody "I usually get the chicken but I'm getting the lobster because enigma is paying, ha ha ha!" in a way that shows they are clearly taking advantage.
I guess the one other thing I would object to is if someone got really badly drunk on my dime if ordinarily they would not get obnoxiously drunk if they were the ones paying. Other than those things, if a guest is otherwise polite and really is enjoying what they get on my dime, I think I would find no problems, and in fact, be pleased, especially if on the way out they said how much they enjoyed the food, drinks, whatever. IMHO as usual.