I’d say they’re acting out of sheer desperation. For some reason that makes sense only to Kim and the higher party and military officials, they’ve got their national identity wrapped up in this need to be a nuclear power. They’re willing to starve their own people and run the country into the ground to achieve such a goal, and if they fail, then oh well, then they can bring the rest of the country with them on the proverbial deathride.
I am most certain that the US has invited them into the international trade environment. The idiots refused over and over. They have isolated themselves and therefore have nowhere to go.
Pretending to use a capability we would displace 7000ft over their nation is quite ridiculous.
One other point is the United States owes them nothing. Let the almighty communists take care of these idiots.