In a live game that I played, the Japanese overextended his fleet and Russia built a minor in Korea. If the Japanese fleet would have been destroyed, would have been easy to build a transport then take some Jap islands with the help of US fleet! :)
But yeah the rule says, quote from book: 3 IPCs for each original German, Italian, or pro-Axis neutral territory that the Soviet Union controls. Theme: Propaganda value and spread of communism.
So pretty straightforward rule IMO, Japan territories are excluded