I'm an Australian and I think I can answer for the majority of us, we find Americans very passive-aggressive and consider them impolite due to this. I work as a screening officer at Sydney Airport, and the vast majority of Americans come across as very fake. I can think of 100s of examples but the worst ones, that literally every American does goes like this:

American: "Hello there was just wondering if you could help me find my gate?"

Me: "Of course. So it's just down the corridor, on your 2nd left..-"

American: Walks off in the middle of my sentence without saying a word and acting as if I don't exist

Or the classic:

American: "There's nothing in my bag sir!"

Me: Finds a knife

American: "Oh my gaahd how'd that get in there haha saahry!"

Me: "That's alright I'll just need to-"

American: cuts me off in the middle of me speaking and starts walking off "I'm going to go now"

Honestly I wish we had a TV show about it, it's so interesting to see how different cultures act, every culture has their quirks but in terms of Americans, they seem to have 2 very separate personalities (all my co-workers agree with this without fail) . I also find in general (outside of work), they're like this too. So to answer your question, I think people find American politeness fake because the majority of the time it is fake.