Some people believe that being a journalist, and thereby enjoying the lawful protections guaranteed to journalists, requires press credentials. It is important to attack this belief whenever it is suggested, as it endangers all those journalists who are not fortunate enough to have a regular salary and a large media group backing them up.
If you take your camera out, and point it at something happening in the street adjoining your home, and plan to post the story to YouTube, Twitter, Instagram, Facebook, your own web page, send to your local television station, insert into a blog or print book, or have any other form of intent to publish, then for the purposes of exercising your rights, you are in that moment a journalist.
There are no special rights that Clark Kent enjoys as a staff reporter that Peter Parker cannot invoke as an independent freelance photographer.
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I believe that specifically, the NDAs in question have a clause requiring personnel who intend to publish books to allow the agency to perform a pre-print review, to ensure no accidental spillage of classified materials through publication. Due to the nature of the contracts, the sole arbiter of whether information is classified is the classifying authority, so even if something has been published internationally in a globally-recognized newspaper, and is now common knowledge to hundreds of thousands of people, the information is still classified until the authority declassifies it. Those subject to the NDAs are technically not even allowed to read that article in the newspaper, or that chapter of the book, unless they then immediately take steps to control the classified in it and report it to their security contact. Which is to say that you are supposed to wrap the newspaper or book up in an opaque cover and drop it off at your security office.
The actual security apparatus has evolved, but the legal framework is still very much from the 1950-1980 Cold War era, so if you do not do the patently ridiculous thing, you can be sued or prosecuted.
Snowden didn't give CIA, NSA, DIA, or whomever else he may have had NDAs with, the opportunity to edit his book before going to print, for obvious reasons, so he's in violation of the contracts. It is unlikely there is anything in there that has not already been published in newspapers. It seems like they're refusing to declassify information that can no longer seriously be considered secret, with the potential to cause harm to national interests, solely because as long as it remains classified, they can use the contracts to silence, suppress, or retaliate against the whistleblower.