Writing in 1982 Ron Rosenbaum offered an interesting hypothesis for Deep Throat's refusal to come forward:
One possible explanation of the silence of Throat … is that if Throat were, like Peterson or Glanzer, part of the Justice Department prosecution team, the disclosure of his identity might give all the Watergate felons cause to petition for a reversal of the verdicts on the grounds of prosecutorial misconduct.
Now that we know Felt was Woodward's source, we also know that he had even better reasons to stay hidden. But Rosenbaum's point is still a good one. Lost in the debate about Felt's "higher duty" to protect the FBI, is the possibility that he was violating the rights of criminal defendants.
Very few anonymous leakers reveal governmental wrongdoing on the scale of Watergate. Sadly, interference with due process is much more common. I noted one egregious case here. Richard Jewell certainly was a victim of this sort of game. Wen Ho Lee and Steven Hatfill may be two other examples.
In these cases, shield laws do not protect the public from official misconduct. Quite the opposite. Because reporters can protect their governmental sources, shield laws make it impossible to punish officials who violate court-ordered gag-rules or who spread misinformation that can taint the jury pool.