I don't know what duties and obligations Fitzgerald had in confirming Plame's status, however, to charge anyone with leaking her name, he would have had to confirm that the leaker knew her status. Could this be why Libby wasn't charged with the alleged underlying crime (he didn't know her status)?
After all, the point of prosecutions is to bring criminal charges.
How does a prosecutor know what charges to file unless he first knows what crime has been committed, if any? How can he determine whom to charge? Not only whom to charge, but what charges to file?
What's he gonna do? Walk into a court of law and say, "Ladies and gentlemen of the jury, I don't know if a crime was committed, but I want you to convict this feller over here anyway." To me, that would be the very definition of a malicious prosecution.
By his own admission, Fitzgerald did not determine if leaking her name was a crime. He didn't do the most basic and important part of his job, but he futzed and fuddled around for two years until he managed to find a way to bring perjury charges against Libby. That stinks to high heaven.