They are a backwards and desperately poor country, true, but they would be no pushover. All of the military evaluations are that an attack by NK would penetrate the DMZ and take Seoul. SK and the USA would win, eventually, but with many many casualties. They have vast artillery reserves in range of Seoul right now - the DMZ is very close. They could open an attack by shelling the capital. So taking out NK with military action is not a very attractive option. I don't even mention what they might do with the nukes they probably have if attacked - shoot one at Tokyo? Could be rather unpleasant.