I remember a few years back, around the holidays, they kicked a guy out (or nearly kicked a guy out out) of the MK because he looked like Santa, wearing something akin to Santa's "work clothes" (not the coat, but red pants white shirt and suspenders) and made no effort to correct any confusion when kids asked their parents "Is that Santa?" He started to create a bottleneck situation, people couldn't get around the crowd that was forming, and he was given a choice between changing into something less Santa-like or leaving.
Now imagine if the REAL Santa Claus came walking in, unannounced. There'd be bedlam!
:lookaroun
Look, either way, there's some suckitude for us regular folk. If you're on line for an attraction, and your wait becomes longer because a celebrity and his posse get to go on first, that sucks. But if you were trying to get to your meal reservations and you can't get there because an A-lister arrived to the park, doesn't have an escort and the hoards of gawkers are in your way, THAT sucks, too. In both situations, you paid your money which is just as valid as anyone else, and someone else has been determined to be more importnt than you and infringing on the vacation you paid for.
But what trumps everything in my opinion is crowd control and safety. Yeah, I couldn't give a rat's keister if Justin Beiber, or whatever the hell that Haley Joel Osment's clone's name is, is at WDW the same time I am. But if I'm walking down Main Street and suddenly he pops out of a store right next to me, and his shrieking fans rush him before I get a chance to scoop up my baby and they knock her over or hurt her, you can bet I'm going to go feral and knock down anyone near my kid until Security gets a better handle on the situation. So it's probably to everyone's benefit if that sort of risk is neutralized from the get-go.