I would go with the the fact that oil and fuels are a global commodity. Since the majority of the rest of the world in using diesel, the commodity price of diesel just follows demand and is higher. Most of the gas that Europe gets out of a barrel of oil is shipped to the U.S., and some of the diesel distilled here, including a good percentage of biodiesel, goes overseas to meet that demand.
Fuel taxes do not account for the spread between gas and diesel prices. I operate commercial trucks and deal with quarterly IFTA fuel taxes. The Federal tax is the same, the states vary. When you factor out the taxes, diesel is still higher than gas.
As for one station raising it's prices and shortly the others follow suit, look to the fact that they are all usually getting their fuel from the same pipeline terminal, thus same pricing within a few cents. Also, virtually all states have regulations that do not allow for huge pricing differences between stations in the same geographic local. Mostly these regs came out of the days when we used to have price wars on fuel. That is the real politics.