Is wearing something as a costume somehow worse then wearing it for clothing? By that logic wouldn't all costumes be offensive to someone? Do you think most people wear some feathers because they are making fun of native americans or because it's just fun to wear? Does it really matter which? I can wear whatever I want because we live in a free society and while you tell me I can't because it offends you, you cannot and should not be able to force me to not wear it because it offends you.

In our society you get to poke fun at all cultures and symbols if you want too. It doesn't matter if it offends someone, because anything can be offensive to anyone. The most offensive thing you can do at most Halloween parties is most likely not dressing up. There are costumes that might be taboo in our culture right now, but you shouldn't be fighting to make MORE costumes taboo, you should be fighting to make them all socially acceptable. When you draw lines in the sand and say "You can't do this because it offends me" you just create a divide. People cannot stop other people from being offended, but those being offended can learn to let it go. If you set a standard that something being offensive is enough reason for it to not be acceptable in society you open up a can of worms that allows any group to persecute another by claiming it offends them.