Anonymity.
According to the law, which Fitzgerald is apparently better at keeping in mind than Libby was, prosecutors, investigators and jurors are forbidden to identify anyone mentioned or testifying in a grand jury investigation. Only people who are indicted may be named. Nobody else.
The rule is, "indict or be quiet".
On the other hand, as Fitzgerald mentioned, any witness may call a press conference and divulge all or any part of his or her own testimony without penalty.
If Official A would like to tell all, no one is stopping him... or her. He gets to remain anonymous because the laws regarding secrecy in grand jury investigations protect him and because he himself is not interested in shedding that anonymity.