I was truly shocked to discover this when I moved to the US. Up to then I had considered butter in a sandwich to be one of the pillars of civilisation. However, I immediately recognised the deliciousness of mayo in sandwiches, and the wisdom of Americans in this regard.
I now use butter only on a baguette, on soda bread and brown bread, or on toast, melted. Though if anyone is ever in the great state of Wisconsin and looking for a superb fast food experience, I highly recommend a Culver's butter burger.