Because the idea of one person's freedom may inhibit someone else's idea of freedom. Such as ISIS, they are a radical islamic group. Therefore beheading Christians, and mutilating women and the like. So I think that their "freedom" to behead anyone who doesn't believe in Allah kind of would detract from someone else's freedom to worship whoever the heck they want.
Is this a concept that you can't grasp? Because if I had a bit more freedom around here I'd be tempted to push that ban button.