Because it's pointless? People that have feelings and drives that make them do heinous things won't stop having them, and won't stop seeking others having them and discussing them. They'd just publish their manifests on other places. There are tons of public places. Let's say next psycho creates a Github account and publishes the next psychotic rant as Github repo. Now we have to burn Github to the ground? Or only if there are three such psychos that know how to set up a Github account?
> The point is that some opinions are just wrong.
True enough. The problem here is that somehow you think you can always tell which ones, and that you will wield the power to do it. The experience shows neither are true - you probably hold lots of wrong opinions without knowing it, and the power to exclude wrong opinions from polite society probably will be not in your hands. The best way to check every law would be "what if my worst enemy was in charge of implementing it?". If you're still OK with it - then it's a good law. Otherwise you're assuming Powers That Be would always agree with you - and that's a dangerous thing to assume.