After their destruction of Cambodia, why did the USA leave it to Vietnam to liberate and feed the Cambodians?

The Khmer Rouge could not have gotten into power without the sustained (and highly illegal) carpet bombing of Cambodia by the USAF between 1969-1973. It took the Vietnamese to throw them out, and for the first few months only Vietnam brought food in to the place. How shameful is it that the world's then weathiest nation could help destroy a country for no reason, and leave it to its almost equally impoverished and ruined neighbour to help it?