Simply?

Wearing costume warbonnets promotes racism and dehumanizes an entire race by stereotyping all Nations into one and reducing that entire race into a costume to be worn and discarded as desired.

Stealing sacred objects for the sake of “cute” photos is not only offensive but racist; it suggests that the spoiled wants of an individual is more important than the people of the cultures being stolen from. It is racist because not only did the settlers come here and commit genocide (and many other atrocities) but then their descendants think they can steal our sacred items too.