That might be a false correlation indeed, however I disagree when you say: "there's very little to think that level of service in restaurants in the US is related to the expectation of the tip"

waiters with good work ethics will offer a great service no matter if there is a tip involved in the end or not. Those with poor work ethics however would care less about providing a good service. I didn't mean to sound insulting to waiters, a lot of them do a great job, but I think that for some waiters the financial lure is important.