Anytime you try and deliver a naughty potty joke, you're taking the chance of offending many people. Shit humor is difficult to pull off, especially when it's in the form of a bumper sticker AND has an actual picture of a stinky pile of POOP to further emphasize it. Some might say the poo pic is an example of this sticker taking it all the way, but I disagree. It only adds REJECTION to an already mediocre joke. As I sat there reading it I shook my head and refused to accept this bumper sticker as funny, and also refused to accept this guy as part of my community.
Now, had the sticker read this instead: "I'm only speeding cause I already shit my pants" AND included a picture (I'll let you use your imagination) then my reaction would have been entirely different. While I personally would never put something like that on my own vehicle, I would at least have been completely shocked and laughed in disbelief because of it, and the true intended result would've been achieved.