Japanese are a mixed bag like Americans are. The Japanese can be profoundly polite and I find them to be utterly mystified when an American performs a small social courtesy for them like holding a door open or maybe helping them make change at a cash register by handing them a few small coins.
The Japanese are generally only rude when moving within the anonymity of a group. Tour groups in Hawaii will demonstrate this is you've never seen it.
Americans are similarly a mixed bag. Things are better than they were 40 years ago when the WW2 generation was going around the world putting the truth to the term "Ugly American". But you still run into the occasional moron who's out there misrepresenting the USA.
Yet when pressed even the most rude American will still demonstrate sometimes remarkable charity. I won't cite what I've seen but I will say that I have seen an American get arrested in China for helping a woman escape from a burning building because the authorities could not understand why anyone would do such a thing. On the upside of this I did heinous damage to a thug who was trying to rob an elderly man in Beijing and the cops didn't believe him when he told them that I had intervened. I was let go and I got the heck out of Dodge ASAP before they changed their minds.
The world is just a funny place and everyone is different. Some of them are dangerous, too, so don't think I'm one of these silly 'tolerant' types who thinks that we could all get along if we just 'understood' each other.
We can't. There's evil in the world and sometimes there's just no living with it.