The U.S. Supreme Court will hear a case that could result in vast swaths of the U.S. being tax-exempt for treaty-protected tribal commerce.

At issue is whether the state can tax the import of motor fuel by a Yakama Nation tribal member from Oregon to his on-reservation gasoline station—Cougar Den. Or whether it is shielded from Washington taxes on wholesale fuel by the Yakama Treaty of 1855. The treaty reserved and secured a range of rights for the Yakama bands,...