I can only comment on the establishments I frequently visit

Waitstaff in Japan offer significantly better service than the USA, according to a Japanese friend or two.

Qualify what you mean by poorer service. Just because the service is different doesn't mean that it's worse. I'd take a waitress that was curt and left after I'd placed my order over one that asks me if I need anything every 10 minutes.

Can you show us some data about it?

Really? As someone who is from neither of those locations, but has been a tourist to both, I've had the opposite experience.

Totally cultural. Visitors from other countries might find the US waitstaff to be overbearing while US visitors to other countries may find the waitstaff to be inattentive.

No, they don't really offer poor service, but their tolerance for jackassery is a lot lower.

If you're rude to the owner, you're toast. "The customer is always right" does not apply. Maybe you're wrong, and they'll tell you to your face. "White wine? With your steak, sir?" can be absolutely dripping with disdain.

I think it's better that way. They can be more honest and not have to pretend they actually like you. The way they treat customers they don't know might border on hostile, but they will go to great lengths to accommodate their regulars.