Actually, Federally-recognized Native American tribes have a government-to-government relationship with the United States and ARE seen as being sovereign nations.
For all the good it does them ...
Like I stated, so is California and Texas. The Constitution grants only the Federal government the authority to "coin." There is no exemption to Native Americans outside of treaties. If they had a need for their own currency, they would already have it.
Ummmm, they do mint their own currency.