So what? Those communities existed underground for decades and have only recently been pushed back into the mainstream. Being underground means it has less visibility to the mainstream and is thus less likely to influence people.

Again, serious, professional white nationalist terrorists or whatever will always have clandestine means of communication. But potentially violent nutjobs are only likely to become violent when they get egged on by other nutjobs, and by reducing the size of the nutjob community we reduce the threat.