Ever actually visited 8chan? Most of the boards there keep threads on-topic and at higher snr, than, say, reddit.

Moderation is the process of keeping a board functional and snr high.

Censorship is the process of suppressing certain types of content from the board. In this case legal content.

You can have high quality discussions on less-moderated boards, e.g. any of 4chan's interest boards. And you can have low-quality discussions on highly moderated boards, e.g. all of Reddit default subs.