The KMT were not communist. They were also friendly to America. They also had democratic aims.
You seem to be misinterpreting the actual radicalness of communism and the communist party in context of the time.
There was one group called the Dixie Mission (from America) who preffered Mao though. Those people were later purged. John Paton Davies and several others were bona-fide communist sympathizers.
That's why the US supported KMT. What I mean by "good" is "friendly to America"
The DPP in Taiwan used to be called "communist sympathizers" by KMT. After DPP took power, it started calling KMT "communist sympathizers". Sounds like "communist sympathizers" are everywhere.