[baron_harkonnen]

NK has a very complex relationship with China and I'm fairly sure they would not be "wiped out" immediately.

Even in the extreme case of North Korea nuking Japan, it would still lead to an extremely complex international situation. The US retaliating for Japan with a nuclear strike or any similar situation of extreme force would be a far too risky of a move.

Not only is NK important to China strategically as a buffer between themselves and the US friendly South Korea, but the Chinese people feel a kinship with the people of NK, not entirely dissimilar to the way the US views Canada culturally. Aggressive retaliation would unquestionably make China an aggressive and active enemy of the US (or whoever decided it was their job to retaliate).

North Korea's proximity to China, not just physically but culturally as well, makes the issue far more complex than if a rogue nation where make similar threats.

[Symmetry]

North Korea is currently hoping that their ICBMs mean they can toss around nukes in their back yard while not having to worry about the US nuking them back. If they can take out enemy seaports and air bases they might possibly have a chance against the conventional forces already arrayed against them in the Korean peninsula. It's, well, it's a terrible plan but maybe credible enough to dissuade the West from thoughts of regime change.