Free speech thrives on open debate. The more confident you are of something, the stronger the reasons for it ought to be. And those reasons must be able to defend themselves against contradiction. When debate is silenced because its mere existence threatens the strength of that which has been accepted as a foregone conclusion, emotion has triumphed over reason. This is how free speech dies.
Along the same lines as the adage, "when you tear out a man's tongue, you are not proving him a liar, you're only telling the world that you fear what he might say."