Say you've got weed in your car, and somebody hits you from behind-- it's not even your fault, you weren't speeding or anything so it's not your fault but a cop is called down. He arrives at the scene and notices you have pro-cannabis bumper stickers on your car. Even if it's not your fault, the thought of a DUI arises and your more prone to a search as opposed to somebody who didn't have the sticker on.

Look, expressing your beliefs is extremely important, but a weed bumper sticker just raises a red flag. Officers are still people, and wouldn't you be more inclined to search a car sporting drug-advocating slogans?

But that's an extreme example and chances are, things like that won't happen. The problem isn't that it's harmful, the problem is that it doesn't benefit the cause. Telling people at a red light how you feel makes you seem like an imposing person. If you truly want change, you need to get involved with real activism-- bumper stickers not only deter people from your cause, it increases your odds of a search. It might even be probable cause.