Uhhh.... You do know that if nukes are launched at North Korea, they WILL retaliate. Would you really want to see Los Angeles or New York City nuked?
I'd be hard-pressed to see North Korea reach LA, let alone NYC, with their current ICBMs. Maybe some Alaskan town at the furthest, but that'd be a waste of a nuke as is.
In truth, they present a nuclear risk akin to that of Pakistan, which for all their sabre-rattling is a rather localized problem limited to neighbouring countries. Of course, unlike Pakistan, North Korea's immediate nuclear shadow doesn't fall on a neighbouring nuclear power but rather on a handful of non-nuclear nations, backed by a distant nuclear power that vastly outreaches them for the time being.
I thus find it more plausible at this point in time that a North Korean "second-strike" retaliation won't involve nuking the US, but rather South Korea and Japan instead. This is actually more worrisome, as it would result in a much higher body count than a direct retaliation against the US, and it stands within reason that both South Korea and Japan, who I'd expect to be aware of this threat, would consider the development of adequate anti-missile defenses to be in their best interests for survival.