you cant wear your wedding attire into the wdw themeparks unless your private event is planned there through dftw. also some resort restaurants too, you should always call ahead to verify if its okay to wear your wedding gown, as at some restaurants a dftw bride might have booked a private event & will take precedence on whether you can come in your bridal attire or not like at california grill, ive heard of this happening here on the board.
as for the reasoning, i think disney doesnt want brides running into eachother, they like each to feel special. ntm the confusion with the princesses like others have mentioned.