OK, those are some white guys where Native American headdresses. Who cares?
If the answer to that is you, please explain how anyone is harmed by their headwear.
Because they are trying to claim something that doesn't belong to them. Not all oppression is about physically harming someone. When a culture absorbs another culture and makes it into a fashion statement, it fully demeans and debases that culture. The significance of the Native American headdress holds meaning to tribes that it does not hold to white people that wear it either as a piece of fashion or as a decoration while they get drunk and scream at a football game. White people have already made a concerted effort to subdue the influence of Native American culture in the United States by essentially isolating them and just treating them like they're a burden. Now that people are going to take the aspects of their culture that they find conveniently fashionable whilst sweeping everything else about them under the rug, they're getting even more drowned out because they don't even have claim to something that was purely their own anymore.
So to answer your question: It's not physically harming anyone, but it's playing into a long standing tradition of treating them like sewer rats.
I don't really know what to say to this. Unless you can give me a concrete example of how actual Native Americans are harmed (doesn't have to be physical) I'm not buying it. I've been to an actual Native American reservation and believe me, they've got bigger problems than white guys wearing hats that most tribes never actually wore (shame on you for stereotyping them like that). The Sioux were the only tribe to ever really wear those things. Do you think that members of other tribes are appropriating Sioux culture when they wear headdresses at pow wows?
Your viewpoint (well, I have a hard time believing that its really "yours" but whatever) is also ahistorical in the extreme. Very few cultures today would even exist if it wasn't for people adopting and fusing aspects of other cultures with their own. You seem to be advocating for some kind of cultural segregation where no culture ever changes or syncretizes with any aspect of another. There's a lot of rich cultural heritage from many different cultures that wouldn't exist if people like you had been in charge.