Its boil down to whoever win on the battle.
The winning side get to decide the meaning of free speech, the meaning of radicals, right or wrong or where the line is.
Yes, society as a whole has a collective opinion that forms the mainstream and defines a) what's outside of that and b) what to do with those outside of it. Do you see a problem in excluding these opinions from mainstream platforms like Facebook and reddit? Even with a pretty open stance towards free speech I see excluding people posting racist death threats from a platform I use to look at cat pictures as a positive.
I see moderating who can participate in a group as part of the opinion that this thread seems to try and uphold. If reddit (as the more or less democratic whole, not the company or technical system) prefers not to give terrorists, white supremacists or whatever fringe group that is based on hatred a platform you're free to see that as a problem, it's just not one the mainstream agrees with.
Likewise, the "extremist" too have their viewpoint, to them you are the danger, and they too will fight for it.
It's almost a Poe's Law of moral relativism in general, but there's the additional step of "and therefore, I will not have opinions on right and wrong and will let others do as they please, because who am I to have moral opinions about others"—but that in itself is a moral opinion about others.
The Overton window (which is what you are referring to) changes over time. What is considered acceptable is transient (I've seen the term for ethinic minority change 5 times in my life time so far and the older terms are now considered racist). Homosexuality for example would have been outside of the Overton window in the past and would have been deemed outside of polite discussion.
People have lost their jobs or banned for comments on Facebook / Twitter etc that are literally years old. Most of these comments were made made over a decade ago.
There are many groups of people that conduct a witch hunter using "outrage archaeology". They will go through someone's tweets for something that is now considered inappropriate and they will demand an apology for things that were said decades before and the person probably doesn't even remember why they made that comment at the time. Bernie Sanders (who is considered from my understanding to be somewhat of a progressive candidate) had something he wrote in 1980 dredged up in an attempt to smear him.
I've was temporarily suspended on twitter because I posted a Death Threat to my friend as a Joke between us and some other friends that were following each other. My friend and I regularly trade racist remarks about each others race on whatsapp because it is a long standing joke. Should we be censored because of an inside joke between friends?