It's not culture hoarding so much as not wanting people who aren't part of the culture to do something tacky with it.

It's hard to explain how it feels to an American, because there isn't a lot that is sacred in American culture, but the closest analogue I can think of is the whole "fake gamer girl" thing. I've noticed that men who have been members of video gaming culture for a long time tend to get pretty irritable when attractive young women start wearing Mario or Portal shirts with no knowledge of or respect for where the images on their shirts came from. They also get pissy when said young women say "omg, i'm such a gamer!" because they like to play candy crush on their phone. They get downright angry when said women start acting like they're longtime fans of comic books, when in reality all they've seen the latest avengers movie. Said women usually end up getting a lecture on how wrong it is to appropriate gamer culture just because it's trendy right now or because they think it will make them more attractive to men. The same type of woman is often the subject of lectures from those involved in nerd culture, too - people decry them for calling themselves nerds when they don't have any real understanding of how hard it was to be a nerd before it became trendy, they never got bullied etc for being a nerd, and so on.

It's a bit of a frivolous comparison (given the gamer/nerd are subcultures rather than actual cultures), but it's not dissimilar to how someone from a marginalized culture feels when their culture is appropriated by hipsters who want to look sexy and interesting."How come this instagram model is getting paid to wear nothing but a native american headdress and a thong? She doesn't even know what the headdress means or where it came from. My people used to get murdered and discriminated against for wearing that, and now suddenly someone from a totally different culture is getting rich for wearing it?" Etc.

Cultural appropriation is a difficult thing to be wary of because it's so variable, but I can at least understand why it pisses some people/cultures off. It's not because they want that thing to be all theirs, it's more that they don't want this important cultural thing to become something that trendites do for scene points.