(Forgive the link source - I can see in hindsight how that might not be perceived as the most objective source - but there are many other sources for those who care to look.)
The problem in these discussions is that radicalize is an overloaded term. It could mean convincing someone of your non-mainstream political view, or it could mean inspiring someone to commit an act of terror. The former seems perfectly fine to me and well within freedom of speech and association.