The issue would be like for example if you were playing as the Shawnee for example and you really only had the US or Canada to transition into. I'd imagine a lot of Native players or even those that want to feel more culturally consistent would rather transition into another more culturally consistent group than a settler colony that now controls most of the land once occupied by the earlier cultures.
I have a strong suspicion that Tecumseh's preorder bonus is actually a surprise package with the Mississippian and Anishinaabe. I think that maybe
why he is a bonus leader and how they were able to represent the Shawnee in a sensitive light: they have their own three act structure that progresses regardless of US and Canada. Obviously US and eventually Canada could be options, but if my hunch is right, this particular issue with our Native American civ will dissipate with knowledge that Tecumseh comes with a full three civ suite.
To put this in another light: we are speculating approximately 13-14 civs per era (I actually think we will have a few more than that in modern era to tie up loose ends like Australia, Gran Colombia/Mexico/Brazil, Morocco, Ethiopia, and maybe Sweden or Safavid Persia). Translating that to Civ VI terms, that means we are getting approximately 13-14 "full civs" across the entire three eras. Obviously there is a bit more work involved in each "third" of a civ this time around, but we can still draw the rough analogy. Civ VI though, released with 18 full civs, plus Aztec as a preorder bonus. If we
only got Shawnee as the preorder civ, we are kind of getting cheated, because translating that to Civ VI ratios we are only getting a
third of a "full civ." We are getting a third of what we got with the Aztecs. Consequently, I think the likelihood that the Shawnee come as a "full civ," one antiquity (Missisippian), one exploration, and one modern (Anishinaabe), makes the preorder bonuses seem a lot more equitable by comparision. Put another way, 1 bonus civ on top of 18 felt a lot meatier than 1 bonus civ on top of 39-42 civs will feel. Ergo, I think Tecumseh has three civs.