That's a fairly common thing at WDW. They don't want your apperance to take away from any of the magic from the other guests. I have seen Bride and Grooms who come in full dress and tux asked to go and change. They don't want guests to be confused with a guest that are acting the part vs the cast members that are putting on the real show. I understand it might look bad at first glance but they really do it because they care about all the guests. It's also common practice that when they do have to send someone out of the park for this kind of situation that they do something special for the person being asked to leave. Such as free park tickets, fastpasses, lunch, ETC. The best way I can explain it is you would not let someone come on stage during a performace and disturb all the hard work of the actors (cast members) that has been done to put on a GREAT show. Not to mention the security issue this causes. Can you imagine how you would feel if a child was harmed by someone that was acting like santa clause at Disney then come to find out that he does not even work there, what would you think about Disney and their security then?