You know, I'm not American so can't quote for sure, though I'm very surprised no-ones mentioned what I think is the real reason, is they're just doing it out of racist spite for the Native Americans involved. Cannabis is a billion dollar industry which I'm certain Trump gaggles to hold, whereas Native Americans just stop oil pipelines being built - he probably hates them more than the Mexicans lol! I bet next year when it's in some white, middle class venue it'll suddenly be fine!
And whoever said about the fear factor is right too, it's classic throw some muscle around so he can charge for the same protection later. It will get reclassified too I reckon as soon as thinks of a way to get patents sorted for all his pharmaceutical corporate chums.
I may be wrong, as I don't know the guy and his team well, and possibly they are religious nutted-up enough to let moral convictions outweigh profit, but I'd still be surprised if so tbh. I think cannabis will be safe, it'll just be more charged for every associated aspect of it!