And Fallout like to dive for the most exaggerated. So if there's a chance something could have gone super-Native American it would have. Either as a joke or a possibility.

After all, Native Americans in general are pretty far and thin these days but you get a lot of emulation of the Native American image. This may be a predominately South-West thing, but Caesar's Legion is composed of hundreds of forcefully annexed tribes that had Amerindian motifs: Blackfoot, Painted Rocks, Twisted Hairs, Twin Mothers, etc. And you'll get similar sprinkled throughout.

Unless they were scouting Seattle for a game setting then they may have gone back to the contemporary military set-up or urban gangs for names. But something like Yakama County with its name, history and rural set up it's a pretty fresh and tempting move to stray from organized urban branding (Tunnel Snakes, xxth [whatever] battalion) to something as unfamiliar but near as adapting native american branding. It's less demographics and more how the Fallout brand works.