Because this group shares a particular political belief of domination by violence. It is a political and social movement, not limited to religion. If they go to the mosque, they hear this message every time they go. If they go to a community center, this is what's being discussed. I know that you look around and you see your doctors, people that you trust, and you think they can't possibly be part of a religion that preaches violence against unbelievers and infidels. They are, they just don't want to be violent themselves.