I could be wrong but I believe since Native Americans are considered citizens of their own sovereign nation which is inside the USA, they can make their own laws regarding anything that happens on their land.
Reservations aren't their own Sovereign Nation.
Their land is still considered part of the US' sovereignty.
They are still under US Federal law, but they are exempt from State Laws.
There is no Federal Law against Casino's therefore they are allowed to operate Casinos on reservations.
I'm not sure, but I believe they do have to pay minimum wages due to that being a Federal requirement.