I've never been to the
MNSSHP so I'm not sure but I'm guessing that there is still meet and greets going on so say you're dressed up as cinderella and you look like the real deal, then when some kid is meeting with the real cinderella and he/she sees you walk by, the kid could get pretty confused. This could be one reason why they don't want people dressing up as Disney characters too well. Also they probably don't want you saying that you're a Disney employee then doing some unspeakable thing that gets Disney into trouble.