Cosplay has become a staple in most popular culture events that gather more than a handful of people, but regular parkgoers will know that the Walt Disney Company does not allow adult fans to dress in costumes while attending their theme parks. While they encourage children to do so, it's explicitly forbidden for adult guests (which for the sake of this particular rule is apparently locked at the age of 14) to dress up as their favorite characters. Indeed, the act can get you not just removed from the park, but banned for life -- and a recent write-up at Reader's Digest reminds you why.
They Disney Parks rulebook specifically notes that guests can be denied entry if they are wearing a costume or mask to a Disney Park, with exceptions for special events where costumes are encouraged, or for people who get specific permission from the park. The Reader's Digest piece notes that it can be a little difficult to get to the exact nature of the rule once you move beyond the masks, though, noting that "Masks are only appropriate if they are necessary medically for your stay at the park, but wigs, tutus, and, of course, the iconic mouse ears are fully acceptable under the Disney-approved dress code."
There are two basic reasons for this -- and unless you are particularly galled about not being able to bring your costume to Disney you likely can guess what they are. Facial coverings and the kind of play weapons that cosplay might encourage are both banned from the parks for safety reason -- whether it's loss prevention or the safety of younger guests who might be susceptible to kidnapping, you don't want somebody walking around with a mask when you need to be able to quickly spot them within the park. The weapons rule is strictly enforced even in events where costumes are allowed -- so if you're there for Mickey's Halloween party, go ahead and dress up, but leave your weapons at home.
The second reason is that Disney wants to be able to control the encounters fans have with costumed characters inside of the park, to be sure that they are "on-brand." If you are running around dressed like Cinderella, but you don't have a contract with Disney that stipulates how you must behave or you can lose your job, they can't be sure that other guests might not mistake you for an employee and have a negative "customer" service experience. Nominally, the rules also suggest that this could ruin the illusion for some younger guests.
A fun example of this is on the sketch comedy show The Awful Truth, hosted by Michael Moore back in the 1999 and 2000. An episode featured his animal mascot -- "Crackers, the Corporate Crime-Fighting Chicken" -- barge into one of the Disney parks to investigate claims of poor working conditions for the actors at the park. Moore, his camera crew, and Crackers managed to somehow get into the park, but were quickly escorted out once Crackers popped up in costume and started talking to guests about workers' rights. You can see that sketch above.