Chris Meserole is exactly right about the need for comments. No one loves getting trashed in the comment section, but it comes with the territory. Most of the time, you learn from the comments. Imagine a magazine that didn't allow letters to the editor; that's what the Huffington Post is now.
I do disagree with Chris on one point: I doubt that advertising is the reason. After all, comments drive traffic, which is all that advertisers really care about, and I'll bet that THP traffic will decline as people grow bored of being passive THP readers.
No, I suspect the site-masters are worried about hurting the egos of the celebrities allegedly posting. Let's face it, they're fading away already; they have better things to do than blogging, like having lunch and taking meetings. If they don't like the comments, they can have an assistant read them.