Exactly... no problem. You could go even further and say 'Mohammad demands murder'. Current event would support that. And the muslums would not be able to discredit you.
Possibly true (though I think the muslims would find a way to "discredit" any criticism), but, you must consider the medium of the message.
This is a sign - not a persuasive essay or speech. It is meant to attract attention and elicit further consideration. It simply cannot contain a lengthy discourse or argumentation.
I believe the sign is successful in that it is attracting attention and eliciting further consideration. This thread is proof of its success.