in real life, as well in any GC games ive played where im the allies invading japan, the japanses dont stand a chance of getting their massive army in china back to the home islands because of the massive naval superiority the americans have by the time they're ready to attack the home islands.
if i recall correctly, in real life the japanese had a million or more men stuck in china while the home islands were under threat of invasion. the japanese could of really used those men at home to repell the expected american attack. unfortunately they had no chance of getting them there because of american blockade.
i guess the ai should at least make the attempt, but if your a competent allied player, the attempt will be in vain.