Why are we afraid of employee comments?

I am currently working on an internal blogging package for my company. Besides the standard blog pieces, it contains some special communications features such as digest and on-demand emailing.

I have been fighting for allowing employee's to comment on items posted through the site. However, there is a great fear from our HR group. To help, I have made comments disabled by default and can be turned on by message, however right now, I am also working on a moderation option.

So why is HR, or executives for that matter, so afraid of employee's having the ability to comment on company news? What ideas do you have to help sell having open comments always turned enabled on our site?

I would love to hear from everyone!