I don't like duck at all, and I've seen it on plenty of menus without sweet sauces, but I'd expect that the sweet sauce thing comes from the Chinese influences on the US palate.

After all, "they" even named a super sweet sauce "Duck Sauce" so "we" probably figure, "That's what you eat with duck." Chinese ducks hanging in windows is how most people over a certain age ate their first, second, and many subsequent, ducks. Ok, the ducks weren't Chinese, but they were prepared in Chinese restaurants.