There are plenty of left-wing subreddits with equal measures of vitriol. Somehow they avoid earning the same opprobrium that the right-wing ones do. This apparent double standard creates the appearance of bias.
/r/anarchism is a slightly more complicated case. They do advocate violence in a "bash the fash" way. But their intended victims are often those who advocate for genocide, ethnic cleansing, and other racist or misogynistic methods. Their violent earnestness is also often mocked by the rest of political Reddit.
/r/LeftWithoutEdge was a sub formed for serious discussion for lefties without resorting to violent speech. It doesn't get enough traffic so I urge any leftists / anarchists here to visit it occasionally.
Edit: I should add that one reason /r/anarchism and /r/socialism are still on Reddit is because they took the warnings of the admins seriously. /r/anarchism dialed their rhetoric back after being told they were on the verge of being banned. Most of the banned right wing subs ignored the warnings, and often doubled-down.
/r/strikeaction
/r/militant
are some of the more obvious choices.