Ur missing the point violence should absolutly be policed. Words ideas ideology hell no let isis, ms13, the communists, the nazis, the vegans etc etc etc say what they want. They are all extremists by some definition let them discuss let them argue and the second someone does something violent lock em for the rest of their lives simple.

What you are suggesting is the policing of ideology to prevent future crime their is an entire book about where that leads to said book simply calls this concept thought crime.