People put bumper stickers on their cars for all sorts of reasons, none of which are any business of the cops. Write your speeding ticket and move on.
Still, there was nothing illegal about asking if he had weapons in the car, bumper sticker or not and since the sticker stated, "if you can read this, you are in range". What would that mean? "Watch out for me, I am prone to road rage and have a gun"?
I'm not a lawyer like you, so maybe you can tell me if seeing a gun between the man's knees is probable cause.
It may be PC to believe a crime is being committed, improper transport of a weapon in a motor vehicle. That aside, it is MORE than a legal reason to order him out of the car, and or draw a weapon on him.