The characters are pretty deep. Not only the main characters, but the side characters like Gale too.
When he cooked Meth with Walt, Walt asked him why someone with a personallity like Gale's is in this particular business. He said he is a libertarian and his position is that the government has no right to tell people what they are allowed to put in their bodies. And that if he doesn't make that stuff someone else will and he at least knows he's doing it right and that his meth won't kill anyone (directly).
So the bumper sticker makes sense. I know that many people claimed that the producers wanted to make Ron Paul look bad. But you could only see the sticker for a fraction of a second and if you didn't know it already you couldn't even recognize it. So I think it's just a funny easteregg.