As members of a 230-year old democracy, there is deeply ingrained in most Americans a fundamental belief in the rightness of fairness. This premise is manifested in everything from our judicial system, which emphasizes the importance of a fair trial, to the Fox News Channel, which labels its news coverage as fair and balanced. It's even written into our Declaration of Independence. The notion that "All men are created equal" means nothing if the principle of fairness is not applied.
Given the circumstances described, it's easy to see why the win-win philosophy of negotiating is so widely accepted - at least in North America, where democracy prevails. In fact, it borders on being politically incorrect to even suggest there is merit in any negotiating philosophy other than win-win.