I have taken many pics of peoples reptiles at many shows, I first ask for permision, and have actually been denied several times, for unkown reason, But I have never been asked for copies of the pics. I personally wouldn't want pics of some random person holding my snakes, what would you want that for..? ...lol 
Most of the time vendors are cool enough to let me take pics of my favorites, most people drive and make vacations out of thier trips to shows, so to have pics of what you did on your vacation is natural. I don't think its really RUDE to ask for a copy, but don't expect one...I think if I was asked for a copy of the pic, I would just keep moving and not take any pics...Just my opinion. I have also been asked to not post pics (in a very polite manner I might add....I was explained that many people who take pics of certain peoples animals and then post them on forums like this, many times the pics don't come out and don't due justice to the animal, I respected that vendors request and even thought I took many good shots of his animals I did not post any of them.
Some people have just plain out said NO PICS...and I say OK and keep walking...Shows should be fun, and not confronational, At least in my opinion. I think its all in how you phrase the request, poss. ask for a copy rather than demand one, Chances are you will never see a copy...either way, but To answer your question I don't think its rude if you ask in a polite way rather than demanding a copy..and I personally would never ask for a copy cause you probably won't see one, and why would I really want one anyway..
SOrry for the long post..Too early need more coffee..LOL 
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Executive Reptiles
Amanda Kingsbury & David Kendrick
www.executivereptiles.com