I think you're really idealizing these subreddits here. The way reddit works, the downvoting of dissent will never be seen, that's the problem. (Generally a pro of reddit, but not in this case). Dissent will get sorted out of the comments section by default, and the arguer banned. The nature of these extremist groups is not to have an open forum for discussion.

There is not a public record to be examined and ruminated on, on the contrary it's a situation where you can go and see "oh 151,400 subscribers to fatpeoplehate, this is normal behavior", or (more likely) people who never view comments will just start to see jokes about killing the obese on their front page feed.

There's a difference between 'saving people from unpleasant speech' and providing an echo chamber to hate speech along side the company's incredibly popular other content.

I don't see the connection to antivaxxer logic, but I'll give you the benefit of the doubt there?

Just to be clear I'm not advocating that we ban all 'unpleasant speech' from the internet, but I do think siloing it off so you really have to be looking for it works, and that there's significant evidence to back that up.

In reddit's case in particular, many of the banned subreddits were using smart tactics like brigading and vote manipulation to amplify their numbers, and push the 'this is normal' message even further, although that's somewhat orthogonal to what we're arguing about.