Why did the native americans (spoiler)

The tweakers and the natives were two possible death scenarios/endings that Mike and Chris managed to sidestep. So what we see happen instead is both of those possible endings play out in a way that cancelled them out completely. Unfortunately for them, this new ending proved unsatisfactory for the entity that oversees all this so Mike and Chris are left to an ambiguous fate.

It is clearly a plot device, as others have said, and basically just one of the possible "endings" to their story. From a realistic standpoint, I'm going to guess that the head guy (Joseph or something? I forget) was bitter about his life and resented non-natives in some kind of way. He was working some nowhere job as a security guard at an indian casino (stereotype much? haha), so it makes sense he gets off somewhat on power and authority. He was obviously sociopathic to a fair degree and pretty much only cared about getting some extra money.

We don't know the history of the area either, but it seems as though their properties and land was used fairly frequently by drug addicts, homeless and other such people stuck on the bottom rung of society. Perhaps some of the older experiences with these types turned violent or something and the native american dudes just didn't give anyone else the benefit of the doubt. Plus, many of these people can simply "disappear" and have no one really notice. Basically a stereotype of "reservation justice", at least in the mind of this jerk, haha.

Because he had agreed to five days, and his fifth night was up that night. It was a way to make sure the cabin was, indeed, clear. The dude already expressed his contempt for squatters with drug addictions.

Because, as indicated with the dialogue at the start of the film, the Indians have a zero tolerance policy in junky squatters ... with bodies buried in the hills, etc.