The problem here is that Natives are offended by it. If a Scotsman was offended by an American wearing a kilt (I am going to swallow my distaste at the fact that we are comparing the situation of white people v. oppressed people to white people v. white people) and asked that American to stop, because it was disrespectful to Scottish culture, I would hope to all hell that they would. But when Native people ask white people to stop appropriating their sacred traditional dress, more often than not they get “It’s a free country” “I’m APPRECIATING your culture” “It’s not racist (because white people get to decide what’s racist now? :/)” “You’re overreacting” and my personal favorite, “You’re being racist against white people!” There is an ENORMOUS lack of respect in this field.

But! There is also a difference in context here. Cultural and racial oppression do not exist in a vacuum! As counterintuitive as it may seem, equality does not mean placing every issue on a level plain and pretending that you can compare Issue A to Issue B and Issue B to Issue W, etc. It’s about looking at the issues in context, understanding all of the nuances and historical backing and intersectionality of those issues, and working on correcting injustices within that context. One of the context issues you need to understand when considering cultural appropriation is that Native people have historically been - AND STILL ARE TODAY - severely oppressed and abused by white people! So when white people co-opt very, very deeply significant Native culture, treat it like a fashion statement, and refuse to respect Native protests, it is an enormous slap in the face on top of thousands of other daily grievances. 

You call allowing people to dress like this a “liberty,” but cultural appropriation is just another form of oppression. Asking that people refrain from disrespecting Native culture is not unreasonable, it is not censorship, and it is not infringing on anyone’s rights - it is asking for respect and understanding.