I'm not sure this was true. The US might have expected to take such large numbers of casualties IF they insisted on invading very quickly for purely political reasons. But given a bit more time, there need not be so many casualties at all. Japan was literally starving. Everyone was on severely reduced food rations, including soldiers. Within a year or so, no one would have the strength to resist invasion anyway. In addition, the blockade destroyed Japan's ability to produce armaments, so the only way to resist invasion would have been wooden spears and hand to hand combat.

A man who's subsisted on 800 calories a day for the past 2 months won't put up much of a fight. Wait a few months more and he'll be dead, at which point he really won't put up much of a fight.