I actually saw a station in the middle of Yosemite Park, that was charging $5.35 / gallon... in July 2007.
I can beat that. $6+/gallon in the Canyonlands National Park/Needles in Utah, October 2006. They were closed and thank goodness we had enough to make it back to Monticello.
The middle of Yosemite is an even more captive audience than Lake Buena Vista. Whatcha gonna do? Argue?
But I DO far less blame the Park Service for that gouging than WDW because of the transportation in getting the gas there, and the volume(s) that would be delivered vs an easily accessed and highly populated area like Orlando.