I think it's incredibly naive to think that this has nothing to do with North Korea. It's extremely likely that this group of attackers is North Korean, and just as likely that they are under the North Korean government's employ. Also, they threatened attacks on movie theaters on american soil. The fact that the movie playing would be (distantly) owned by a Japanese company is irrelevant, the American moviegoers are the ones that would suffer.
That said, I think canceling showings of the film sends a shitty message, and sets a bad precedent.