Autonomy for Native Americans is based solely on legal agreements made long ago. It has nothing to do with culture, faith, or guilt. If there was a way to duck out of those treaties, we would already have done so.
So certain Californian Tribes are legally permitted to cultivate and preserve magic mushrooms because of 19th century treaties...???
1 - hardly any of those treaties stood whenever the whites found a valuable resource on the land that had been previously given in treaty to the Indians.
(just read about Deadwood Gold Rush and how quickly the US Government found a way to get out of that treaty)
2 - In the 19th Century NO ONE CARED ABOUT ANY DRUG USE - Anyone could grow mushrooms, or weed, or buy Heroin and cocaine from the local shop.
In fact one of the first prohibitions was stopping alcohol being sold to the Indians (research origin of term 'BOOTLEGGER')
but absolutely no one suggested laws prohibiting 'drugs' before the 20th century.
MANY Native American legal exemptions are based on First Amendment FREEDOM OF WORSHIP issues.
Which are, by there very nature, cultural and spiritual reasons.
And white guild DOES play a huge role in the US Government backing off the Indians on issues that would be strictly enforced against other groups.