<devils advocate>
The NSA is arguably having a credibility problem at home in the US. It needs to convince the tech industry that there are enemies abroad who threaten their security, and attacks by nation states (who aren't the US) is something that is real.
North Korea is a great scape goat. They can deny it all they want, and we don't care about the diplomatic costs, because it can't get any worse. People can't demand sanctions, can't demand recall of ambassadors, the only thing anyone could demand would be going to war with them, but for the most part, we don't care. They're the crazy uncle of the world stage. So basically, the only problem would be if the NSA got caught lying about it.
</ devils advocate>
However, getting caught would probably be the worst possible thing for the NSA (remember, there is likely still a leaker inside); as it would jeopardize the main benefit from doing this in the first place. So I don't think the risk versus reward pans out. That said if North Korea IS behind it, the above motivation for speaking out is still valid.
Also one was supposedly raided by the FBI at the end of October but that story was never updated.
Regarding encryption bans I've mostly seen justification referencing the Charlie Hebdo attacks (which are assuredly not a false flag).
Personally I believe that North Korean sympathizers were behind the SONY attacks given a number of pieces of evidence, but most heavily the #GOP leaks of emails detailing SONY collaboration with the US State Department and RAND Corporation that point toward The Interview being a strategic diplomacy product.