Personally I don't think the "usual and accustomed" argument will hold water. Reservation boundaries would be clearly defined and only within those would these LEO's in accordance with what has been mentioned above.

They can certainly claim that these hunters were a danger to the public and they intervened as such, but they better make a damn good case of it to convince any reasonable jury or judge that this was indeed the case. It doesn't seem as though such claims have been part of their "defense" thus far. Truth still matters regardless of whether they carry badges or not.