They are protecting themselves, legally, as they deem necessary. Here’s Cheyenne River Sioux Tribe Chairman Harold Frazier’s response to the governor.

They have the right, native Americans suffer more from this and have a right to protect themselves

I believe in and respect the sovereignty of the Indian nations.
I am sad to report though that Oklahoma Indian Nations often support Republican politicians with an Indian heritage even though the Republican platform in Oklahoma says they do not believe in sovereignty.

The tribes are considered a sovereign nation and can often set their laws. Given the circumstances they are doing the right thing!

That governor is one piece of work. She has no authority over their land and they are protecting their people. She just wants votes come election time. Leave them alone. They are doing more good than harm and she's just a power hungry control freak like all the rest.

The Tribes have no 'treaties' with US States, only with the US government -- and the US government has broken most (all ? ) of those.

They are doing the right thing. Sadly I think things will go badly for them.