Look, let's be honest: they should've cleaned up after themselves.
Let's also be honest: they shouldn't have been attacked with dogs, blasted with loud music and blaring lights, rubber bullets, sprayed with water in sub-zero temperatures, etc.
Does the second excuse the first? No, of course not. But it does paint a clearer picture.
As to why they didn't clean up when the protests were over, my understanding is that they were more run off than leavning in an organized manner.
I don't want to spread false information, so here's my disclaimer: I could be totally wrong on that last point and would be more than happy to be corrected. I'm fully aware that my Facebook feed is an echo chamber, but frankly given the context of the authorities' responses to the DAPL protests, I have no reason to doubt it.