The Am chord can be seen as an ellipsis of Am7 alias C/A. In my post above, we can assume the same melody C B A G above a constant chord C with alternating bass notes C and G - absolutely normal in European-American folklore.

On the other hand, Pete Seegers was a singer-songwriter with a contemporary message, so he was well entitled to use whatever accompaniment he deemed fit. Even "ordinary people" among Pete's contemporaries were used to maj7 chords. Objections would more likely come from Folk Police mindsets, but, as we all agreed, not entirely unsubstantiated.