I grew up in the mid-Atlantic region of the U.S. and saw it both ways. Butter on sandwiches just sort of seems like just another way to make one, like some people like Mayonnaise (I don't) or not. I'd probably put it on a jam sandwich, or a ham and cheese, but not a PB&J (the Peanut Butter takes on the same role).

Of course I also grew up in very immigrant heavy areas so that may have been part of it.