It is not just the current administration, it has been every administration since the Korean War unofficially ended (technically, we are still at war). Previous administrations (minus Clinton and Bush 41) supported Japan and SK from the Commie threat posed by NK, China and Russia, plus the fall of countries in SE Asia to Communism. Democracy had to be present in that part of the world somehow.
After they were nuked, Japan completely renounced war. They barely have the ability to carry out a preemptive strike on NK let alone actively go to war with NK and fight on Korean soil. The Japanese Self-Defense Forces are well equipped to defend the homeland, not fight overseas. I suspect that will be changing in the near future, especially in light of Japan's problems with China and NK. It took a long time for Japan to organize and properly equip military and police forces so they signed a defense agreement with the U.S. That agreement covered external and internal threats. U.S. servicemen and women were actually used as a police force in Japan until the country could be rebuilt and demilitarized then somewhat remilitarized. Only the external threat part of that agreement exists now, I believe.
I have noticed many young South Koreans have an anti-U.S. attitude but middle-aged and older citizens are very thankful. The U.S. is pulling the majority of its troops out of South Korea, however. All it takes is a few NK missiles or tank shells and that attitude would be wiped away fairly quickly, though. Perhaps that is what needs to happen?
Like it has been mentioned before, NK has nuclear warheads but they do not have a proper delivery system (missile) for them. Iran, on the other hand, has a proper delivery system but does not have warheads. Both would not think twice about giving these weapons to Islamofacists, which have added another battlefield to their already long list: Mumbai, India. I would not put it past either to use them, as well. That is why neither can obtain the appropriate technology.