Prosecutor Fitzgerald made it pretty clear that he isn’t prosecuting for the leak itself because he can’t, and that is the source of the Obstruction of Justice charge. He was obstructed from getting to the truth. From the Patrick Fitzgerald Press Conference Transcript (you’ll love where this is posted):
And what we have when someone charges obstruction of justice, the umpire gets sand thrown in his eyes. He’s trying to figure what happened and somebody blocked their view. As you sit here now, if you’re asking me what his motives were, I can’t tell you; we haven’t charged it. So what you were saying is the harm in an obstruction investigation is it prevents us from making the fine judgments we want to make.
And later in the press conference,
What I’m simply saying is one of the harms in obstruction is that you don’t have a clear view of what should be done.