They shouldn't go butt naked to this event for the same reason I shouldn't go butt naked to Best Buy. I'll get kicked out. Why? Because I entered their store, thus expressing my wish to use(and in a sense am already using) their facilities. At this point I am agreeing to adhere to their rules. If their rules are that I must be clothed then so be it. The exact same thing happened to this girls(more relating to the modeling agency that provided the costumes though) there were rules, they didn't follow them, they got kicked out. Know if they feel like continuing to dress that way when they leave the convention just for fun, no one will stop them.
To be honest with you, I hate pants. I really hate pants. They are a deterrent to all things good in the world. So when I am in my own house, I don't ware them. However, when I am agreeing to go to someone else's home, where their rule is that I must wear pants, I do. It's about adhering to the rules set fourth by others when I am using their things, just as I would ask them to adhere to my rules when they are using my things.