Turns out I was giving him too much credit. He's just making a smug (non-hybrid related) political point and happens to be driving a Prius.
He passed me a week or so ago and pulled away so slowly that 14 miles later he was still in sight. I thought I had him pegged then, but this morning clinched it- I was behind a dump truck and he passed me to sit behind the dump truck. His point? We each have a bumper sticker expressing our opinions of a particular politician. Our feelings about this politician are somewhat different, and his feelings have to be in front.
Yes I'm annoyed, but not by the fact that his views are manly enough to pass a car whose engine is off. It just hurts to watch someone making stupid passes while still getting much better mileage than me.