It seems to be endemic in certain US circles to confuse Communism with Authoritarianism Why are the two things so regularly conflated by Americans ?

Because the American education system and lifelong propaganda peppered throughout popular culture creates this confusion intentionally. It’s like a word association game: anything not capitalism = oppression and death. This is why, in large part, why most people will describe capitalism when asked to define communism or socialism.

So this is more of a ‘how’ vs ‘why’ answer, however I’m sure you can easily arrive at the ‘why’ using the explanation I’ve given.