Snowden warns us that we're being spied on and that the grip the NSA has on the whole Internet goes far beyond what even the most paranoid had imagined and the US government answer is: espionage prosecution, international warrant etc.
Compare with: Unnamed NSA official, no doubt with the blessing of his bosses, anonymously reveals the same kind of information about NSA spying - but this time because it is convenient for the administration and it fits into their political agenda, there won't be any legal consequences, prosecutions etc., absolutely nothing will happen, we all know it - and even worse - we all passively accept it.
Laws are being selectively enforced by the government; there are no actually classified documents. There are "things the government wants you to know", those can be leaked and released on demand by "unnamed officials" - screw the legality of it - and there are "things the government doesn't want you to know", and anyone revealing those things will be spied on, harassed and prosecuted (James Risen? Laura Poitras?), it doesn’t matter that the people writing about those are journalists who have no duty of any kind towards the US government, they’re just doing their job.
If the administration has proof of North Korean involvement, they can present it to try to convince the American public... but wait, no they can’t. They can't do that because the evidence they have comes from the NSA exploiting and hacking systems all over the internet. "Yes, your honor, I saw it all, it was the North Koreans who painted that graffiti. How do I know? I was there that night, burying a few bodies in the empty lot next door".
The NSA giving actual proof of NK involvement is equivalent to them coming forward and admitting what they are: a threat far more dangerous for the security of the Internet than anything North Korea will ever be capable of.