Someone on Facebook questioned this position:
A "chief" riding a buffalo is close enough to a brainless savage. How else to tell the story on the cover? Put them all on horses or motorcycles. Show the Batmobile racing against a pickup truck. Or do something other than the "riding" motif. But don't make the Indian look like a primitive beast-master.
Note also the barren desert background with the fantastic rock formations. This is typical of the land in northern Arizona and southern Utah. In contrast, the South Dakota landscape consists of rolling, grassy plains.
The message is clear. Indians live in remote, barren places because they live remote, barren lives. They're "out there" somewhere, choosing to live in isolation. They're not part of the American mainstream.
That's what most people believe, anyway--even though 70% of Indians live in urban places. Our mythology tells us that Indians are wild in every way: wearing wild costumes, communing with wild creatures, and living in wild places. They're wild, primitive, and savage.
For more on Grant Morrison's comics, see Indian in RETURN OF BRUCE WAYNE #4 and Chief Man-of-the-Bats.