In all honesty, are ideas such as "it's impossible to be racist towards white people" and "doing anything any non-western culture does is bad because 'cultural appropriation'" the norm in social justice circles on Tumblr?
I know this is Smurfette's thread, but since I'm in what is considered tumblr's social justice circle, I think I can answer this.
The short answer is yes to the first one and no to the second one.
This is the long answer.
Social justice bloggers operate off of the definition that racism = power + prejudice. Racism is simply bigotry and prejudice with the overwhelming institutional power to back it up. This does mean that, in Western society, only white people can be
racist, but people of color can still be
bigoted or
prejudiced. In common usage, all these words mean the same thing, but in social justice circles, "racism" has different connotations.
There's a difference between simply one culture having something in common with another, and actively taking things important to one culture and devaluing them in another to enforce racist stereotypes. The problem with the vast majority of cultural appropriation is that it devalues a symbol important to a certain group (i.e., warbonnets for Native Americans) and turns it into a commodity. It also encourages stereotyping of that culture in the process and perpetuates racist stereotypes. For example, the warbonnets that hipsters wear encourage the stereotype that all Native American cultures are the same. Wearing "Chinese" or "Native American" Halloween costumes reinforces other people's stereotypes of what those groups are like. In the case of "Native American" Halloween costumes, they also (at least on women) have a strong tendency to sexualize, objectify, and exotify Native American women - a demographic group that is already especially prone to sexual violence.