If you've read Blowback, you're well acquainted with the long and less-than-honorable history of the American occupation of Japan since WW2.  If you haven't, read it.  In the meantime, here's the situation to which these protestors object:

Some 47,000 U.S. troops are stationed in Japan, with more than half on the southern island of Okinawa.

Residents have complained for years about noise, pollution and crime around the bases.

Not surprisingly, many Japanese object to this long-term American military presence on their land -- and what looks like their government's possible willingness to sell them out to curry favor with Washington.  The Japanese cabinet will announce its decision in May.