North Korea's military is made up almost entirely of Soviet and Chinese throwbacks. Their economy and production facilities are so broken that if they build a bomb, it's likely to be the only bomb they build until the next time they flex their muscles in a few years, never mind that China has already hinted that if North Korea initiates any wars, they won't back North Korea (but if North Korea is struck first, they will honor their alliance). Hell, even if they build a functioning bomb, the odds that said bomb would have enough fuel to reach even past the DMZ let alone American soil are worse than your chances of winning the Mega Millions jackpot.
North Korea is a threat to itself and very little else, no matter how much they bray and holler before running to hide behind China when called out.
That adage doesn't apply when the country you're about to pre-emptively bomb into a parking lot a) has an alliance with a global superpower which will go to war as promised, b) presents extraordinarily little threat to anyone outside their peninsula, and c) would, if they did attack, do so without China's backing. And given how often North Korea hides behind their alliance with China when someone so much as says something mean to them, this is just another example of the Kim family being all bark and no bite.Better to be safe than sorry though. Thats the american way.