I think there's a difference between "can" and "don't". The difference is intention. The honor student bumper sticker says (my kid) "can". The other one says (I) "will".

I think the honor student bumper sticker is horrible, but it doesn't promise violence--just states that a possibility exists. The "don't share" bumper sticker is a statement of intent, and does promise violence.

Therefore, I think one is acceptable as free speech, and the other is not, in my opinion.

Karen