America had total control of Japan following their surrender, and the time/power/resources to rebuild Japan as they saw fit (which was to become an eastern bulwark of capitalist freedom, against China and Russia).

Bombing Cambodia had the much more cynical purpose of convincing Ho Chi Min that Nixon was an unrestrained madman whose demands in peace talks had to be surrendered to, to avoid further mindless devastation for all involved. Yes, it was more complicated in the details, but pretty damn clear in the larger picture.