Wikipedia uses "a left-wing anti-fascist and anti-racist political movement." Until recently, there was nothing controversial or illegal about being any of those things.
The historical antifa is the German Antifaschistische Aktion. You can read about some of the things those guys engaged in - they didn't just target Nazis - to see why the association is controversial.
"After the Communist International's abrupt turn in its Third Period from 1928, the KPD regarded the Social Democratic Party of Germany (SPD) as its main adversary and adopted the position that the SPD was the main fascist party in Germany. This was based on the theory of social fascism that had been proclaimed by Joseph Stalin and that was supported by the Comintern during the late 1920s and early 1930s, which held that social democracy was a variant of fascism. Consequently, the KPD held that it was "the only anti-fascist party" in Germany and stated that "fighting fascism means fighting the SPD just as much as it means fighting Hitler and the parties of Brüning."