I don't quite agree with this. We need to be careful and not give the terrorist Ctrl-Left good video to use in propaganda, but we can be that careful and still defend ourselves.
When a thug in a mask engages in terrorism (a non-government individual or group using violence to advance political objectives), I'm okay with a forceful response. Even if the terrorist is not masked like the Black Bloc, if an innocent person is struck and the violence was clearly initiated by the terrorist, and if an armed good guy is in position and prepared for an immediate response . . .
If masked thugs show up at OUR events, they must be unmasked, forcibly if necessary. There is only ONE REASON for the masking; to avoid being identified and punished for the crimes that they intend to commit. I'm guessing that few of us ever show up at their events, except in counter-protests separated from their group.