One of the obvious reasons as to why I find these decals to be atrocious: they are just that— they're revolting. It is not socially acceptable for you to pee on things you don't like (publicly or otherwise), so why in the world would you display that on your vehicle?
Another reason everyone should hate these stickers: Calvin is just an innocent comic strip character. In the source of this Calvin and Hobbes scene, he's filling a water bottle. Someone manipulated the image to make him appear to be peeing. Calvin has absolutely no reason to pee on U of L, or Obama, or the Mexican flag, or an IRS logo, or liberals, or on any brand of products that you dislike. And neither do you.
Peeing on things generally doesn't improve any state of affairs; in fact, if you can think of a situation in which it would be helpful, do let me know (and don't you bring up that pee-on-a-jellyfish-sting myth).
These stickers, like many bumper stickers, are nothing but an ignorant display of opinion. If you have one, we probably aren't friends, and I'm more than okay with that.