Because America's agenda is much better than that of the Soviet Union.
That certainly is not clear. While I'm 90% of people would rather live in the USA than the USSR, the USA was not creating a "little America" in Vietnam. No, it was propping up an authoritarian, albeit capitalist, regime, just as in South Korea. There was no American liberalism, even the economy was quite statist.
Even though South Korea EVENTUALLY became democratic, that was not conceived or at issue during the Korean or Vietnam wars. At that time it wasn't communism VS democracy, it was right-wing authoritarian bastards VS left-wing authoritarian bastards.
(though you might ascribe to the theory that certain right-wing authoritarian bastards with sufficient economic growth allow for the creation of capitalist liberal democracies, which may or may not justify support for right-wing bastards over left-wing bastards)
A stubborn porcupine: heredity & nationhood. Meditate, brother!
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