No freedom of speech legislation ever is absolute. There are always limits to what you can legally say. Slander is legal in how many jurisdictions, for instance? I guess Wilders argument is that the Koran amounts to “fighting words”, i.e. a call to incite violence, while his own anti-Islam statements do not. Leaving aside whether he’s right or not, the legal arguments seem to be a lot more involved than he makes it out to be.