This seems to hold true for most broad-interest sites like newspapers and magazines where comments can be downright awful, as opposed to sites like Slashdot with a self-selected and somewhat homogeneous audience.
If you read Slashdot at -1, you'll see plenty of horrid comments. Heck, people can be quite rude in +5 posts, although usually not both rude and stupid. Slashdot isn't helped by being self-selected or homogenous; it's helped by heavy moderation, both by users and by admins. Newspapers and magazines seem to leave their commenters to their own devices more. Rather than modding down the trolls, people reply to try to debunk them.
I normally read slashdot (well, skim) at -1, and the comments here are relatively decent. It really helps that I can browse as a single webpage. Compare slashdot to "news" sites, where the "noise" overwhelms the signal. And since slashdot has (had?) a reputation for tech articles, many of the commenters know how to type.