Laws against anti-semitism lend credibility to proponents of conspiracy theories. "If what I am saying isn't true, they why is the government out to stop me from speaking about it?"
If you look at France in particular the attempt to shut down the comedic routine of dieudonne m'bala m'bala who has a routine that makes fun of the holocaust has likely done orders of magnitude more to stir up anti-semitic sentiment in France than his routine
regarding the anti-islam stuff, its a similar situation, with some important distinctions that I won't get into at the moment. Regardless, silencing anti-islamic sentiment, just as with silencing anti-Semitic sentiment, will only make the situation worse
P.s. its not just in Europe. In Canada we have 'human rights commissions' that can force you to apologize and pay enormous fines and legal fees for 'hate speech'.