People are against it because it's not a very effective form of activism. Gatherings thousands if not hundreds of thousands if not millions of people together, and have them all smoke weed in front of the White House for example, would be true activism.

Putting a sticker on your car merely exercisizes freedom of speech, and not much else.

Honestly, have you ever had an opinion, saw a bumper sticker that preached otherwise and changed your mind?

Be realistic-- bumper stickers can express dissatisfaction, but they don't rally in the streets.

Think about MLK for instance, do you think the fight for civil rights would've been as effective had every black person in the nation placed some bumper stickers on street corners?

It's nice and sweet, but it won't do anything to change the laws which is what we really need to be focusing on.