Even North Korea has nukes, and a sworn enemy even, and no nuclear holocaust.
North Korea's nuclear arsenal is nothing compared to the arsenal the US has. North Korea would only be able to take out one American state at best whereas the US could obliterate the entire country.
Of course, not without a lot of radiation spilling into South Korea and China. The US is allies with South Korea and they do not want to piss off China.
NK isn't about taking out American states, it's about nuking Seoul
China did a similar reminder with their own arsenal - not officially - as unofficially as it gets, scower the Wikileaks for it - but they admitted that their own nuclear arsenal is primarily "meant" for Taiwan (Republic of China)
This makes tactical sense, since NK has sworn to retake the south, and PRC has sworn to retake the ROC - and these ambitions outweigh any opposition they might have against America. If "shit hit the fan" between mainland and insular China, the nuclear arsenal would ensure that the war was as brief as possible, and mainland China would simply step off the boats, and retake the island, without much infantry fighting.
China has also (in Wikileaks) admitted to not giving a shit anout NK, and that if they do invade SK, they will not oppose a western intervention.
So, it still comes back to common sense - NK has the nukes, they have no real allies, they have nothing to lose - they could start launching those nukes on Seoul today, right this moment, but they won't because it's too destructive a thing to do, it backfires too hard. It's easy to think of "pariah countries" as illogical and cooky, but they DO posess a survival instinct