One other thing might be that outhouses were an American innovation[1], imported to other parts of the world later; and thus, of all rural-living subcultures (which are the usual source for a culture's hygenic mores/superstitions, being the most affected by them), rural Americans have had the longest to adapt to not worrying about getting shite on their feet. :)

[1] http://www.radiolab.org/2009/sep/07/sculptors-of-monumental-...