Japanese Government looks after its own good, enacts policies that helps Japan the most, as well as honoring our alliance system. If you know anything about Japanese corporate culture (that is if you ever have the privilege of working in a Japanese owned company), there is emphasis in the word of the strategic management team. Once a promise or oath is made, it is honored. We Japanese honor our alliance system -- and because we are dependable people, some one you can count on through tick or thin -- we stick around for our American friends. Besides, it also beneficial to have US military presence in the region. It keeps China, NK, Russia in check. Its less defense expenditure on our part.
This is one thing I truly admire about the West. Its a collective front. A las, Asia is a much more interesting chess game.