They don't make much money at all on gas -- most of their coin comes from the sodas, chips, and candy bars that they sell. And have you checked the price for a bottle of water? Damn near $2.
So while they're trying to stay competitive, they can (and do) jack up the prices on the stuff that they really make money on -- coffee, donuts, and sundries.
I am looking for the day when they start charging you to take a dump.