Well, as an FB-registered user, you don't actually have a password. So you can't change your email or your password (since there's nothing to verify against). The interface doesn't show this yet. (If you have access to the email address, you can do the lost password thing and that will generate a password for you, though that's roundabout.)
Thus, if you change your email in FB, we really need to change your email here to ensure it's linked correctly.