Don't know why this got downvoted. Communism has killed more than Nazi's, so if we're banning Nazi's because they're a potentially violent threat, it follows that communism/marxist subreddits would also be banned.
Governments of communist countries have killed millions, not communism itself. Communism is a political theory designed to try and support the common good and break a cycle of oppression and revolt. Hasn't worked in practice, probably won't, but it's certainly not a hate doctrine like Nazism.
Glorification of the USSR would be more complicated, and could be perceived as endorsing violence.
The primary element of Nazism is violent racism. This is somewhat unique among political ideologies and makes these comparisons poor. If you ask a communist to describe their utopia, it almost certainly doesn't include millions of people being executed by the state. But ask a Nazi and it will.
... and anyone who doesn't agree with Communism's version of that is an evil person who is promoting the exploitation of the downtrodden, and therefore inevitably will be violently opposed.
That is, though Marxist Communism talks about everyone being equal, it also talks about violence to those who disagree. And in practice, the violence has shown up more reliably than the equality has.
Only because the USA decided to enter WW2 and helped defeat Germany. If the Nazis had won you would be looking at greatly elevated body count numbers today. After all Hitler viewed Aryans as the master race and all other races inferior, and the extermination of the Jews was simply the beginning of his grand plan.
Glorification of the USSR would be more complicated, and could be perceived as endorsing violence.