Not to wade hip-deep into the argument. I do not find it offensive.
The first time I saw the billboard it reminded me of the T-Shirts, like the Homeland Security picture, sold at Pow-Wows displaying similar pictures and captions and wondered how long before that one would show up.
If you look hard enough there will always be someone offended by something. But, in this case I think it has less to do with the mistreatment of Native Americans than it has to do with some peoples negative emotional responses to firearms.