It is pretty simple and I have to agree with Disney on this one. Adults wearing home made feeble attempts at costumes confuses small children. Disney wants to control the character experience so it is magical for the children.

I personally don't get why adults want to dress as characters and go to the theme parks but I have always tried to ignore it but my problem with it is when kids are seeing poorly done costumes or a 350 pound cinderella. We have to explain to the kids, "No that isn't really (insert character here), that is just somebody that thought it would be funny if they dressed like them."