Peter Grant explains the logic. First, he cites George Friedman on why the Trump administration has been unable to do anything about North Korea despite its efforts to bring in China on the matter:
The US turned to China to negotiate a solution. The Chinese failed. In my view, the Chinese would not be terribly upset to see the US dragged into a war that would weaken Washington if it lost, and would cause massive casualties on all sides if it won. Leaving that question aside, the North Koreans felt they had to have nuclear weapons to deter American steps to destabilize Pyongyang. But the risk of an American attack, however difficult, had to have made them very nervous, even if they were going to go for broke in developing a nuclear capability.
But they didn’t seem very nervous. They seemed to be acting as if they had no fear of a war breaking out. It wasn’t just the many photos of Kim Jong Un smiling that gave this impression. It was that the North Koreans moved forward with their program regardless of American and possible Chinese pressure.
A couple of weeks ago, the reason for their confidence became evident. First, US President Donald Trump tweeted a message to the South Koreans accusing them of appeasement. In response, the South Koreans released a statement saying South Korea’s top interest was to ensure that it would never again experience the devastation it endured during the Korean War. From South Korea’s perspective, artillery fire exchanges that might hit Seoul had to be avoided. Given the choice between a major war to end the North’s nuclear program and accepting a North Korea armed with nuclear weapons, South Korea would choose the latter.
With that policy made public, and Trump’s criticism of it on the table, the entire game changed its form. The situation had been viewed as a two-player game, with North Korea rushing to build a deterrent, and the US looking for the right moment to attack. But it was actually a three-player game, in which the major dispute was between South Korea and the United States.
It doesn’t matter if China is on board if South Korea is not. Hence Peter’s conclusion:
This explains, to my mind, why the US response to North Korea’s undoubtedly aggressive moves has been so muted. There is no doubt that the USA could turn the whole of North Korea into a radioactive desert – but that would poison parts of China and most of South Korea with fallout, which neither country will accept. Short of such an all-out nuclear attack, any US military intervention in North Korea must inevitably involve South Korea. If South Korea is not willing to permit its territory, or its airspace, or its waters, to be used for that purpose, the USA is effectively stymied.
I see only one way to break the logjam, and force the issue. That would be for the USA to announce that, in view of North Korea’s aggressive actions and stated intentions to become a nuclear power, it is willing to sell nuclear weapons to Japan and South Korea. Note that I said “sell” – in other words, not station US nuclear weapons in those countries under US control, but give each country its own nuclear warheads and delivery systems, under its own sovereign control. China would instantly have kittens – a nuclear-armed Japan must be close to its worst nightmare, and a nuclear South Korea wouldn’t be far behind that. If anything could force China to rein in the North Korean regime, that might do it.
And the reality is that now it is clear North Korea cannot be reined in by the USA, Japan is going to need its own nuclear deterrent, which it probably required anyhow given the growth of China’s global power. SJWs and Republicans may believe Asians are all a single political bloc, but anyone familiar with Asian history knows otherwise.