At least the military industrial complex got even richer of it. That's the only reason.
It's easy to judge history in hindsight. USA bombed the crap out of Japan during WW2, and Japan had amazing recovery.
Not to say that Cambodia was at all justified (or Japan for that matter), but that it's more complicated.
It's a lot easier to judge a person on objective things, like how effective they were at executing their policy.
Between 1965 and 1975, the United States and its allies dropped more than 7.5 million tons of bombs on Vietnam, Laos, and Cambodia—double the amount dropped on Europe and Asia during World War II.
Pound for pound, it remains the largest aerial bombardment in human history.
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.
Did you happen to forget that the United States occupied and reconstructed Japan from 1945-1952?
https://en.m.wikipedia.org/wiki/Occupation_of_Japan
It helps to be familiar with history before making judgements about it.