It seems roughly analogous to abortion clinic bombers. I'm not sure what you do for these things except force secular state education to hamper extremists from running schools to brainwash people. Even then, unless you say if people whose political doctrines are deemed unacceptable by the state must surrender their children to orphanages or foster care, you're not going to get rid of the problem.
If you asked a poll question in the united states about "How many people are prepared to use force to defend Christianity, (or Christian values)", I bet you'll get a much higher answer.
I'm all for making people embrace a radically libertarian view on free speech. But one of the best quotes i've heard is from a post-Soviet Russian "Everything [good] the Soviets said about Soviet communism was a lie. But everything [bad] about American capitalism was true." Insofar as these peole have been culturally and economically excluded from Dutch life, or modern western capitalist ways of life fail to offer deeper spiritual succor, it isn't surprising that people are turning to their ethnic and religious identities to find succor, and thus think we should have decency exceptions to free speech. The dutch are a cosmopolitan and tolerant lot, which is why that 40% that want restrictions on speech isn't higher.
Cheney's approval rating is like 20%--which means 1 in 5 americans actually like him, or more likely, think they say they should in a telephone poll. You can find anyone who will agree, when asked, to an awful idea or leader if they think their wider political or religious ideologies require them to do so.
I'm of the opinion that western europe needs to figure out how to deal with this problem not because failure means turning into Iran or Tunisia, but failure will give political power to religiously conservative groups whose faith trumps their belief in civil rights, and push the country towards a political-religious dynamic more akin to the United States'.