It's not just left wing subs. Tons of subs ban people for talking outside the group think: right wing subs, non-political subs. Etc. The structure of Reddit just lends to abusive moderators.

I dislike Nazis, misogynists and children living in their parent's basements too much to go to 4chan.

It's not just leftist subs. The right wing ones do the same damn things.

The worst sins on reddit: \#1. Thinking outside the group think. \#2. Presenting facts or evidence that counter the group think. \#3. Questioning the decisions of the clique who are moderators or friends with the moderators. The really messed up thing is that it's STILL better than 90% of the internet. There is no true free speech forum.