The other reason is that, for now, the weblog host can be liable for the comments made on his/her weblog, regardless of how remote from topic, inane, combative, or violent they may be.
New York Times: This Story Stinks
I've often regarded comments on online news stories rather like the trail of slime that follows a snail. I try not to read them, then regret it when I do. A telling anecdote: At a former parish, a member used to complain frequently that I didn't permit comments on The Coracle. He never directly mentioned his complaint to me, however, nor ask me why I had made that choice. Instead, he complained about it on his weblog.