Aha!! It makes sense, finally!
If one were to launch a first-strike against North Korea, one might want to protect themselves from retaliatory missiles. That's the reason.
What they're forgetting is that they could ship a bomb via UPS, store it in some basement in Washington, D. C., and just wait for the right time to explode it. Don't need to do it with missiles.
Dumb to do it with missiles.
And if it were me, I'd tell the US I had to bomb stored somewhere, and to leave me alone.
Arf