The legality of the provenance of the material is irrelevant to the ethics of reporting. After all, leaks like this are not being submitted directly to outlets: they are being handed to the public. To the extent that journalists have to worry about reporting on leaked material, they have the same concerns about publicizing any other effectively non-secret information: do they do more harm than good by repeating something to a wider audience?
The question becomes more complicated when an outlet receives the content of a leak directly, as happened with the Snowden documents: in that case, the material is still effectively secret, and releases must be carefully vetted, because in a sense the outlet is conspiring with the leaker, and is the party providing damage control.
Reporting on public leaks should be treated the same way as reporting on suicides or other sensitive yet non-secret events: they should be news if they are newsworthy, and they should be reported on in such a way that minimizes damage that might come directly from the manner of reporting rather than from the facts being reported.