You just hit the heart of the matter, I think. The liberals probably think we're taunting them by displaying the ribbons on our cars and trucks. Same with American flag stickers, the other thing guaranteed to send liberals into fits of shuddering rage.
It never failed to amaze me before the election. American flags and ribbon stickers were very common on cars with BC '04 stickers. I did not see a single American flag or ribbon on a vehicle with a Kerry/Edwards sticker.
While I did not see a single Bush-Cheney bumper sticker where I live (Staten Island, NY) I have noticed a ton of the support the troops stickers. And Staten Island went 57% for Bush, so it's not a coincidence. I think these stickers really are the stealth BC stickers, so that people can let others in the know be aware about who they support for president, without having to worry about their car being vandalised due to the sticker.