The simple fact is that when Disney allows costumes to be made for rental shops then people can do whatever they want in those costumes. I've seen people do a lot worse than protest in Disney costumes on Halloween and many of them in places where kids could see them.
I'm thinking that they knew they'd garner more attention, therefore increasing the chances of negotiation from Disney, by wearing costumes. I hope they get it sorted out before it goes any further because, as much as I love Disney and am looking forward to our upcoming trip, I won't cross a picket line (if it comes to that, hopefully it won't).