“Leak investigations do not focus on the receiving journalist for a variety of reasons,” according to a 1982 government report (pdf) on unauthorized disclosures of classified information.
One of those reasons is that “journalists are unlikely to divulge their sources in response to a subpoena for documents or testimony before a grand jury, and contempt sanctions against journalists in other types of cases have not been effective.”
In other words, according to this analysis, the traditional refusal of journalists to cooperate with leak investigations protects them in the long run by discouraging government officials from undertaking further investigations.
The 1982 report, known as the “Willard Report” after its chairman, Richard K. Willard, is a minor classic of cold war secrecy. Though frequently cited in the literature, it has not been available online until now (thanks to S).
See “Report of the Interdepartmental Group on Unauthorized Disclosures of Classified Information” (the “Willard” Report), March 31, 1982.