In so doing, they violated the standards of integrity that newspapers and other media have led the public to expect. They knew, but withheld a material fact, in their stories. An excellent analogy would be a crime reporter covering a controversial murder investigation. A reporter who had inside information and knew who murdered Jon-Benet Ramsey would be breaching the trust of the public if he did not disclose that fact in an article describing an accusation of that individual and the latter's denial. I don't see why the press covering the Starr leaks presents any different ethical dilemma.
Reporters make a religion of protecting sources.
Journalistic ethics do not demand that a reporter lie to protect a source, but that he refuse to answer the question.