This sentiment is common but misguided. For one thing, "impressionable people" will always seek out taboo information. Banning jihadi messaging isn't going to make it less appealing to them (when they find it elsewhere) or to make it disappear from the internet (there's always a way). So this is an ineffective strategy.

It is better to target the root cause, which is that jihadi ideas are clearly attractive to a large group of European youths. Just look at the numbers of Europeans travelling to Iraq, Libya, Syria to fight, to sacrifice their lives for this ideology. You can't dismiss all these people as "impressionable". As far as I can tell, they are otherwise normal people who are as rational as you or me.

Sure, there are some truly impressionable people out there. And we should prevent them from being radicalized. But it's like with sex or drugs... We talk to our kids about difficult subjects so they are well-informed and are not so naive and easy to manipulate. But we don't talk about jihadi ideology. Why is mainstream society unwilling to challenge their ideas and expose them as wrong?

You can't defeat an ideology by pretending that it doesn't exist, which is what you're effectively doing when you censor it. It's a knee-jerk yet cowardly way to avoid confronting the actual problem. And the problem isn't going to go away unless you confront the ideology head-on. Radicalization isn't the inevitable outcome of being exposed to these ideas but is rather the result of losing an argument.