Same reason it made sense for the U.K. to prevent Germany from invading France and consolidating Europe. A China fearless of its near seas is a China more ready to secure the energy and natural resources it so desperately needs in central Asia, the middle East, Africa, and South America. The realpolitik angle is that it's simpler to balance a consolidating power versus a consolidated one.

Japan's utility is not only as a strategic launchpad. It also represented, in 2012, $216 billion in trade [1]. That is a $5.5 trillion asset when discounted at the U.S. 30-year rate.

[1] http://en.wikipedia.org/wiki/List_of_the_largest_trading_par...