America is about the rule of law.
And freedom. As in a person doesn’t tell another person how to conduct his/her life, and vice versa, as long as each follows the rule of law.
Many Indian treaties were signed by the government, and then ignored (not honored) and forgotten.
I would not claim that it is a requirement (legal or otherwise) that “assimilation” (consisting in part in dishonoring signed treaties) is a prerequisite to live in America.
America has been about assimilation of peoples and cultures since its founding - assimilation does not mean a Borg style collective, but rather a shared set of values, a common language, and culture.
Most US Indians do not live on reservation for good reasons, and because they are assimilated into America. You would know that if you were an American.
You seem to value separation, Balkanization, of the US which it is definitely NOT about.
“Rule of law” is a difference matter having to do with governance, not to be conflated with assimilation/culture/values/language/etc.