The Justice Department should not be chasing national security reporters. But those reporters have a responsibility, too. As Eric Holder wrings his hands in remorse over his feverish pursuit of Fox News reporter James Rosen’s phone records, it’s worth noting that, when it comes to national security leaks, some things are secret—and should be kept that way—for a reason. In Rosen’s case, the alarm bells went off not because he reported that North Korea was about to conduct a nuclear-weapons test but because he reported that the CIA learned of this fact from a source inside North Korea. In other...