Reddit has huge "tyranny of the majority" problems and many communities spring up to essentially censor the speech of people outside of that community so people in that community can speak their mind without getting downvoted (I.E. censored) by the masses of Reddit. BlackPeopleTwitter has a system where all users that are not verified black users will be censored if the moderators deem it appropriate, which is perfectly reasonable. Is this racist censorship? Absolutely and yet it turns out that's not always a bad thing.
I find much of the discourse on free speech to be an oversimplification to the point of self-parody. I support the intentions of free speech but not a literal interpretation of what it means, which is just never censor anybody ever. That literally doesn't work and can't work, it's not a thing that's possible outside of a thought experiment, you'll end up getting DDOSed by speech.
They do not.
They lock down specific, controversial threads in this manner when outsiders start making confrontational comments. AFAIK this was done because the racist vitriol proved impossible to moderate.