I remember going to Star Wars day at Disneyland and there was this guy dressed up as Ben Kenobi. After we took a group picture in front of the castle, he went over to Adventureland and had his own mini meet and greet with park guests. It took cast members hours to realize that he wasn't an employee and throw him out.
This. People have to realize it's for the safety of the guests, not because they want to ruin your fun or immersion. Actually, Universal does have this rule which isn't explained as well as Disney's, but is the same idea:
(This is under prohibited items)
- Clothing or accessories that may create a false impression that visitors work for Universal
It's one thing to wear a wizarding cape, especially if you can still see your street clothes underneath. If you are dressed up in full wizarding outfits and can be mistaken as an employee, then they may or may not do something. Probably depends on the employee who spots you and their interpretation. I've never been to The Wizarding World, so I don't know what the employee uniforms or street characters look like or if any of them look like full wizard outfits.
Disney takes things much more seriously, probably because they want to distinguish themselves as a safer park, but also could be because of incidents that we haven't necessarily heard about that were serious enough to implement the stricter rules (the whole "he/she is the reason we can't have nice things" concept). Sure, most people just want to dress up and have fun, but what about malicious folks?
Perhaps if you were just wearing a Jedi robe and your street clothes were visible they wouldn't do anything because you obviously don't look like a character or cast member, but full on garb could definitely give guests the false impression you are a character or cast member and sadly there are weird people in this world that would (and have) run with it. Disney wants to maintain the integrity of their characters and not have that ruined by someone play-acting as one and cause an incident that reflects badly on them......or worse.....give a child a false sense of security and....well.....I don't like to think of those kinds of things.