At least in the State of California, although as far as I know it is true in every state. In order for casinos to operate on tribal land, the tribe must sign a 'compact' with the state. The compact is more of a contract between the tribe and the state which lays out all of the terms each will follow and who will pay for what. One of the terms is always for the tribe to follow all state and federal laws in regard to employee pay and taxation. Why else would the state agree to allow the tribe to operate a casino unless they could collect some form of taxes?