The Guardian, like AB has an editorial policy and has to comply with the law in terms of libel, defamation, race and sex discrimination. So it is perfectly proper and reasonable that they moderate the comments section.
I have a friend who likes to troll Daily Mail readers, and his posts are often deleted, so I think the DM also censor comments.
Censorship of posts on a forum is entirely different from Governmental censorship such as preventing the publication of a report for several weeks until after a General Election.