It wasn't a government operation that took down AGIS for hosting Sanford Wallace's CyberPromotions.
It may have very well been Japan....Sony is based in Japan, despite SONY having a presence in America. The various trade groups and “conglomerate groups/(read yakusas)” might not take too kindly to what NK did and may wish to send their own message! (I would quake should Japan ever get pissed at the US in terms of cyberwarfare!) The US may have “helped” of course!