But where do you draw the line?
When it comes to state censorship, to silencing people by government force, there is no "line" to be drawn. It's all wrong, period.
Is it as OK for someone to have a discussion forum where they talk about all the sexual fantasies they have about children, as it is to be criticizing their government? Is it as OK to have a place where people are talking about how blacks and immigrants are awful and how they shouldn't be 'allowed' to live as well as white people, as it is to be discussing what is and isn't good about how TV shows are being written and produced? How about religious extremists promoting violence as a way of spreading their (version of their) 'faith', as opposed to discussion of whatever religious text you care to name?
Yes, yes, and yes, provided that by "OK" we mean "legally OK." I have my own personal opinions about whether various of these topics is socially acceptable, but "socially unacceptable" is not the same as "should be forcibly blocked." Why is that so hard for some some people to grasp?
That doesn't mean I have to let you hold that conversation about disgusting pedophile fantasies in my living room, or that my local bar has to let the KKK rent out their upstairs room for meetings. But on the other hand if I agree to provide a communication service and then start removing content, at best I have a broken service, a defective product.
I don't use reddit, but there seems to be a very simple solution to the problem of subreddits with content you object to: don't read them.
extremists end up being louder than everyone else because that's what extremists do.
"Extremist" is a content-free label. Abolitionists were extremists in the 1830s, and William Llyod Garrison was kind of loud. He was also right.