Dear Rich: I have a question. If someone wants to entertain professionally at parties wearing an officially licensed Disney costume, is that a violation of Disney's rights? If so, why does Disney retain rights to control how the costume is used after it's purchased? I'm so glad you asked. The short answers to your questions are, (a) yes, and (b) they don't retain rights over the costume; they do retain rights over how a character is commercially exploited. (By the way, Disney also goes after those who buy and sell unauthorized character costumes.)
Although it may seem like anyone who buys a costume should be able to wear it and entertain for money, that's not the way the owners of licensed characters view the situation. Perhaps it would help if you step back and consider a few hypotheticals, as lawyers often do. Let's say that someone buys an official Mickey Mouse costume and then uses that to make a movie about Mickey Mouse. I think most people who read the Dear Rich blog would agree that purchasing a licensed costume doesn't entitle you to make a movie about the character. The same would be true for a stage play about Mickey Mouse. Ditto if a nightclub comic created a show built around the Mickey Mouse character (wearing the licensed costume). Entertaining for money at a party is a variation on this. The stage is much smaller, but the commercial intent is the same, and such exploitation -- which has become a popular home-based business -- likely violates copyright and trademark law. There's nothing wrong with wearing the costume for Halloween or attending parties, but when you create a commercial enterprise that exploits the character, the copyright owner views that as an infringement.