The journalist in question is James Rosen, the chief Washington correspondent for Fox News, who according to a recently published affidavit, has violated the 1917 Espionage Act by serving “as an aider, abettor and/or co-conspirator” in a 2009 leak that involved Stephen Jin-Woo Kim, a State Department security adviser that shared confidential information with Rosen.
The DOJ has been permitted by a court to access Rosen's email correspondence both with Kim and with other sources and has used it to demonstrate that Rosen had solicited information from him. Still, that is not illegal, and the press should be able to do it without impunity as the right is protected by the First Amendment.
But the DOJ has chosen to debate that very point, and is trying to make it an illegal and punishable action.
There are probably many reasons why they aim to do so, but the two main ones are that they are trying to charge and convict WikiLeaks' Julian Assange of the same thing (and a precedent would be great), and to make journalists too scared to pursue any kind of information that has to do with things that the administration would rather keep secret (and effectively kill off investigative journalism).
Washington Post's reporter Karen Tumulty has summed up the issue perfectly in a tweet: "The alternative to 'conspiring' with leakers to get information: Just writing what the government tells you."
"We are outraged to learn today that James Rosen was named a criminal co-conspirator for simply doing his job as a reporter. In fact, it is downright chilling," Fox News' Michael Clemente reacted to the revelation, adding that they will "defend his right to operate as a member of what up until now has always been a free press.”
Those who have predicted this coming since the days when WikiLeaks published the confidential information shared by Bradley Manning and was attacked for it could be forgiven for giving into the temptation of saying "we told you so", but I doubt any of them are happy about it.