[crazygringo]

I'd say Americans generally expect butter to be used in situations where it's intended to melt. So we use butter on hot toast, hot waffles, baked potatoes, hot dinner rolls, and so forth.

On sandwiches we use mayonnaise since the sandwich is cold, and the eggy flavor of mayo is really tasty, and cold mayo is easy to spread but cold butter is not -- if you used regular sandwich bread it would most likely tear.

Plain room-temp bread and butter we don't eat much of, but that's because sandwich bread isn't usually very good by itself, not because of the quality of the butter.

I think the butter-with-hot-food thing might also be because we usually keep butter in the fridge. I'm aware some people keep butter at room temperature so it's spreadable, but that's never been popular in the US.