Retracting just the owner means you basically allow anonymous posting (except the mods still know who wrote it, and anyone who read it before the username got wiped). That opens up a whole new can of worms.
People can create an account with just a username and password; we're already effectively allowing anonymous posting.
I'm not fond of such deletions, but I think it's good for people to be able to retract their statements when they realize they don't reflect well on themselves.
Perhaps a better solution would be to allow the original comment to be "retracted" such that the author information is removed but the comment remains, so people can still understand the thread of the conversation.