IMHO, you have to know your audience. I've only been to 2 events like the ones the OP is describing and I was probably a rube because I had many dollar bills in my pocket. I thought I'd need them to buy raffle tickets, 50/50 tickets, charity tip jars, and etc. Any charity event I go to is one for a charity I support and I do plan to donate money. I bring the money I plan to donate and spend it on the silly stuff.
The events I've helped organize have certainly been the ones that want to be sure that nobody leaves with money in their pockets, but that's what people expect when they buy their tickets. I've walked around with kittens in a basket while telling people that it costs a dollar for a squirt of hand sanitizer before they can touch them. Our target audience always seemed good with that because it was for cat rescue.