Japan is a nation of savers and has a massive domestic pool of savings. Also, Japan will drop below investment grade within the next ten years as those savers retire.
America, on the other hand, needs to borrow internationally.
Japan's famous savings rate, just like its famous work ethic, is nothing but a myth anymore. It stands now at 2% in 2009. (and its famous work ethic...no more. 30% of its working population is part-time. look up 'freeter')
America, on the other hand, needs to borrow internationally.