I recently encountered this cultural difference (I grew up in Australia) with my wife (we live and she grew up in the US). My daughters vastly prefer my sandwiches to hers because I butter the bread (on the inside, the way God intended), usually using as little butter, or margarine, as possible. When asked why, I say it's mainly because it prevents the moist stuff in the sandwich from turning the bread to mush, but also because it tastes good.

I think a lot of it comes down to the fact that most sandwiches these days are prepared and then eaten right away, whereas I was brought up in a world where sandwiches tended to be prepared long in advance of being consumed.

That said, Americans will cheerfully slather "mayonnaise" on bread.