The farmers pay the tax at the pump just like everyone else. As I said, the tribe isn't actually paying the tax. The consumer they sell the fuel to is. So everyone who buys fuel in Idaho (with the exception of dyed diesel, which is not legal for use on the highway in licensed vehicles) at the retail level is paying tax on the fuel they buy. The only one who is burdened with paying the tax is the consumer. It is a pass-through to everyone else. Yes the distributor pays it up front, but they pass the cost down the line. So, technically, the only tax paid by the tribe or its members is what they actually buy to use themselves.
By the way, the farmers do not get out of paying state fuel tax just because they are farmers. Any fuel they use to travel down the road is taxable to them.
I don't see how the Gas Stations on Reservation Land are any different than a gas station off reservation land. They both collect taxes, paid by the customer, and remit it to the state. I see no unfair competitive advantage... So, what are you REALLY mad about?