Sociology of Car Bumpers

Something my sister pointed out, based on driving from San Francisco to southern California, and subsequently confirmed by local observations (for example, in the parking lot on my way back from basketball today):

  • When you see magnetic ribbons attached to cars, they invariably have right-leaning political slogans on them: “Support the Troops” and the like.
  • When you see left-leaning political slogans on cars (“Support the Troops: Impeach Bush”), they’re invariably on bumper stickers.

Why is that? Are leftists just a bunch of cheap Luddites afraid of using new technologies? Are conservatives just overprotective of the paint on their fancy SUV’s? What’s going on here?