Yes, I do, and did, darn well expect my child to have decent table manners at 3 and stay in his seat when Mickey came into the room. Did he? You be he did. If he had gotten up, I would have been about 1/2 a second behind him putting him back into his seat. Even when we ate at Whispering Canyon, both kids were expected to behave. I absolutely allowed them to have fun in the spirit of the restaurant, they enjoyed getting ketchup, but when they weren't in the stick horse race or delivering ketchup, they were in their seats. They don't have to sit 100% still and only be seen and not heard, but they don't get to run around like little crazy people and cause problems for other diners either, and it's perfectly acceptable to expect that other parents have the same consideration.
This isn't an issue of the kids not behaving, this is an issue of the parents doing nothing about it. Even if your 3 year old gets up when he/she sees Mickey, you, as the parent should be telling them they need to stay seated so that Mickey can come see them. If they don't, they you, as the parent, need to put them back in their seat, if they still don't stay seated, then you, as the parent, need to give them consequences for not behaving properly and remove them from the environment. It's rude to the other diners, rude to the other kids who are waiting patiently to see Mickey, and rude to the character and the handlers who now have to figure out how to still provide a good experience to the people they are supposed to be interacting with while also dealing with your little brat who has been allowed to wander around unsupervised.