Well SB there's 7 reservatons here in Montana, and I can tell you for certain that they are way more sovereign than us. They have no taxes, abide by different laws, have casino's hunt when they want, etc etc etc. They even have signs on the lake that say "ONLY TRIBAL MEMBERS ALLOWED ON BEACH"
I could go on but thats a whole other topic.
The tribes, for any practical discussion are completely beholden to the fed. They are only "sovereign" to the point the feds let them be, until they decide they're not.
As to the constitution, you need to read a little further down in Article I section 8, the part requiring purchase of any land and only when agreed upon by said state legislature, and only for very specific purposes.
