I don't think anyone believes NK is capable of a large scale attack on the U.S. This was a business decision pure and simple. Theater chains made this decision for Sony, and they did so because of liability issues and a broader concern for economic well being. If they decided to show the movie and something did happen at even one theater, it would be a nightmare. Lawsuits, a big drop in business...
Even if nothing happened, ALL of their business would take a hit around the holidays because some people would inevitably stay home with that empty threat looming. Winter break is a huge money maker for theaters. They don't want to have attendance drop for other movies because they decided to show this one.
As for Sony, if there's one thing they're actually afraid of, it's not an American attack. Remember where Sony is headquartered as a corporation and the history between NK and that country.
