I suppose they do it to avoid embarrassing situations like that one that happened with Google months ago, when a worker charged against "Sicko" (we commented it here in a long, interesting thread), or those other employees who make radical statements about their company while using the corporate blog.
Corporate blogs, by its very nature, tend to be quite boring and they read like a press release. A personal blog of a worker, like the ones hosted at MSDN blogs, for example, is something very different (thankfully).
(Why press releases have not been casted away from the face of Earth? They're so amusing... nobody believes them