As long as the comments are clearly delineated from editorial content, I don't think it makes a whole lot of sense to hold the paper responsible for the content of the comments.
How do you figure comments differ from the opinion column of newspapers, which have always very much been the editor's responsibility?
Newspapers are fundamentally different from forums like Slashdot or Reddit - they have a well-defined role in society not as bulletin boards, but as authorities, and part of why that is, is exactly that they have skilled journalists choosing what is fit to print. And this is why editors are public figures.
But there's better ways to prevent that than holding newspapers legally liable for comment content.
Yes, but they are not liable because incentivizing responsibility us a good way to deal with bile, they are liable for the content because their editor is publishing it on their site, a point which bears making.