And what is objectionable about those claiming to adhere to its principles.

IMO, first problem is that it is based on a myth. That if only there had been violent resistance to the nazis they’d have never come to power. There was lots of violent resistance to the nazis - from communists especially. Nazis politically exploited that, spun it into propaganda, and were helped to power by it. Consider fascism more broadly, and the most successful fascist dictator of them all in terms of staying power, Franco in Spain, was cemented by a violent civil war. So the reading of history Antifa is based on is false.

Worse, who exactly defines who is “fascist” and therefore deserving of violence against them? I know of no independent body making that call. So anyone can technically be a fascist. People become kind of fascist themselves when they appoint themselves cop, judge and jury like that.