Except Disney themselves promoted the idea of Disneybounding in the parks with the previous "Show Your Disney Side" campaign. They spent almost two years encouraging people to bound and to further share on social media with specific hashtags and such. So to now act as if it's crazy that people would be upset over the sudden shift, or unsure what constitutes a "costume" is just ignorant. And there is a distinct difference between a bound and a cosplay. Cosplay has never been permitted outside of special events like MNSSHP anyway. But bounding was not only allowed but encouraged for the past few years.
Additionally, events like Dapper Day while unofficial have been welcomed and supported by Disney. I attended my first one this fall. Is a 50's inspired dress "costume?" What about a man in a dress shirt and suspenders? Or a bow tie? Costume or just a certain style? (Because while I may not dress vintage everyday, there are a LOT of people who do.) Disney made special "Dapper" drinks for that day and had signage that matched the Dapper Day website fonts and graphics. So clearly, they didn't mind several thousand sharply dressed guests attending that day.
There is a massively blurry line here. And that's before you even factor in "costume" clothing items as mentioned by
@s8film40 ! There were dozens of Star Wars items for sale last night at the Studios that could brush that "costume" line. Darth Vader dresses, Chewbacca hoodies, Jedi robes - you name it. All for sale. All in adult sizes. The "foolishness" isn't on the part of those having fun at Disney (oh heaven forbid!!); the foolishness is Disney not being crystal clear on what "costume" means after spending the last couple years encouraging creativity in the parks.