Disney's Policy from the FAQs page:
http://disneyworld.disney.go.com/faq/theme-parks/#subCategory7
What is the best way to dress for a day at the parks? Is there any clothing that isn't permissible?
The parks are a casual, family-oriented environment. We suggest you dress comfortably, wear good walking shoes and check the local weather report before you leave for the parks.
Ensuring that the parks are family friendly is an important part of the Disney experience. In that spirit, we ask you to use your discretion and common sense. Attire that is not appropriate for the theme parks (and which may result in refusal of admittance) includes but is not limited to:
Adult costumes or clothing that can be viewed as representative of an actual Disney character
Masks (unless you are dressing up for a particular event)
Clothing with objectionable material, including obscene language or graphics
Excessively torn clothing
Clothing which, by nature, exposes excessive portions of the skin that may be viewed as inappropriate for a family environment
Objectionable tattoos
A Guest is allowed into the parks if her or his hair (or make-up, if applicable) has been made to resemble a Disney Princess or character (for example, after a Bibbidi Bobbidi Boutique makeover) provided the Guest is not also wearing a costume or clothed to look like the character.
My question is, Have you ever been to Star Wars Weekends? We went last year and there were plenty of adults dressed like characters. There was even one lady that guests were posing with and CM came over and made people stop taking pictures with her because she was not an official character with an Disney escort.
I would not have mistaken the lady in question as a princess if you look at her picture, but her intention was to dress like a princess. So, I have to agree with Disney for making her change her outfit.