It's not any kind of exception. Protests involving the klan and the Nazis have often been extremely dangerous.
Aside from that, the point you're making is trivial in an obvious way that you really ought to understand. Violent right wingers are not typically on the protesters' side in American demonstrations, Nazis and KKK notwithstanding, because they are on the side of the police.
Think about what these ideological categories mean for a moment and you wouldn't normally expect leftists to put on riot gear and bust heads for the man, or for conservatives to go out and violently oppose the status quo.