> And who gets to decide which ones are prohibited and the standards used?

In Germany's case, a bench of Constitutional judges, after Guardians of the Constitution informed the parliament that they will prosecute an organization deemed a threat to a free and democratic society. So again, a proper independent judicial instance whose mission is to examinate evidences and make a choice after a fair trial.

Regarding Antifa its more complicated for there is decision-making is distributed. There is no proper leader or leading-commitee, just a bunch of folks working together (and sometimes using dubious means), pretty much like Anonymous.

Also no, wishing a ethnical cleansing is no regular opinion. We aren't debating about budget or public service, but of literal state-sanctioned genocide.