1) They're not logically consistent (big surprise, there) and despite their iconoclastic beliefs, they know perfectly well what is meant when someone treats the queer'an with contempt.
2) A mohammedan view of the queer'an goes beyond it being a mere symbol; not only is it (in their view) the words of their deity, in some ways it IS (or at least is permeated by) their deity. To them, it is both a symbol AND (to some extent) the thing symbolized.
3) Mohammedans go berserk at the drop of a hat. They don't really need an excuse to go off on a killing spree or start a riot ...
Because it works.
Every time they riot we back down.
Now if someone would kill the rioters is mass quantities until they were all dead or fled, we’d rapidly see an end to that sort of behavior.