Yes, it's legal.
When you borrow money, the lender sets the terms.
If you don't agree to the terms you don't get the money.
The reason the lender wants your vehicle titled with the state is because it's the only way that the lender can perfect a lien if you default.
The lender knows darn well that the tribal government would not cooperate with a non-tribal business that wants to take legal ownership of a defaulting native American.
Bottom line: You want the money, title your car with the DMV of the State of Oklahoma.