Why have some of Japan's leaders been talking about the need to acquire the ability to attack North Korean missiles on the launchpads? It's because they know that the United States, despite its overwhelming air and maritime power, cannot credibly threaten North Korea. That is because North Korea holds Seoul hostage by threat of artillery and rockets. Thus Pyongyang's dangerous nuclear and missile brinkmanship is potentially destabilizing.
Moreover, North Korea has given Japan a strong case for developing nuclear weapons with which to deter China, without Japan's having to say so. So East Asia could be on the brink of a spiral toward dangerous nuclear confrontations over which the U.S. would have little influence.
Currently, Japan has no ability to target North Korean missile launch sites. Thus talk from Chief Cabinet Secretary Shinzo Abe about future preemptive strike options is partly domestic politics. Abe, by representing himself as tough on North Korea, wants to cement his claim as successor to the prime minister's post when Junichiro Koizumi steps down in September. But more than domestic politics is involved.