What I find interesting is that many Americans consider themselves Christians and that will influence their decisions. So one argument can be made from that point of view (I'm not religious myself).
In the Bible it says God designated the last day of the week as a day of rest. From a Christian point of view, it would make sense that Sunday is the last day of the week, as it is the official day of rest. Otherwise they disrespect the Bible and skip the "real" day of rest.
I'm not religious or American, so from my relatively objective view it seems as if the people from the majority religion has ignored their holy book.