The reason for that was to break Soviet influence in SE Asia, that's all; nothing particularly 'imperialistic' about it, just Cold War geo-politics. The Soviets were the 'Imperialist' power, not the US.
Nobody was predicting the massacres that came later, and despite all the hysteria, US power is limited by local influences, not all-encompassing domination, regardless of what the loons out in the fever swamps fantasize about. It was a part of the Nixon-China detente policy, actually, Red China not being particularly happy with a Soviet puppet state on their southern border.
Like Kissinger said, most of the time choices are limited and many times all of them are bad, so you have to decide which is the least bad, or something to that effect.
I would like to see an original, primary source for that quote. The earliest I can find is from 1995, and Brzezinski denies making it. All the other references to it seem to be hearsay.
We were still in Viet Nam in 1972; we didn't pull out until 1975. !979 sounds like a better time frame than 1972.