"Cultural appropriation: it's a nice term for stealing, isn't it?"
Sort of, yeah.

"Stealing is wrong, isn't it?"
Yup.

"Or is stealing OK when the people being stolen from don't have enough power to ensure the law will enforce their rights?"
Nope.

"This is why we have so many unenforced treaties in this country, right?"
I think so.

"The indigenous people the treaties were made with were deprived of their legal rights, making it impossible for them to get their promised share of the bargain."
Not sure.  I'd have to study it.  But I wouldn't be surprised if 'deprived' and 'impossible' were true.

"So isn't cultural appropriation just more of the same?"
Nope.  Culture is a constantly borrowed, sometimes stolen, hopefully inspiring and tweaked interactive thing.  If you think somebody's just pinched someone else's  idea, plagiarized an essay, produced a completely derivative piece of art, then go ahead and trash the art for being an unoriginal ripoff.  But if it's good art, it flies.