Sony also has a vested interest in the hack being the work of North Korea. The company is going to be on the receiving end of a dozen or more lawsuits—from employees, ex-employees, investors, partners, and so on. Harvard Law professor Jonathan Zittrain opined that having this attack characterized as an act of terrorism or war, or the work of a foreign power, might earn the company some degree of immunity from these lawsuits.
Don't forget that it also helps them hype their movie as "the move that caused an international incident". I'm sure Sony are quite happy to have this story rather than "we had a bunch of data stolen by someone who was pissed off at us over our shitty business and security practices."