It could be a trained behavior.
Certainly, our means of elimination has running water close by. (If this is what Rob meant by perpetuation, then I apologize). It would be interesting to know if tribespeople from primitive areas have the same tendency.
An interesting story: a college psychology class once trained their psych instructor to stand in the corner while giving a lecture. They used female students' more or less provocative poses as rewards. The instructor was completely unaware why he gradually began to teach from the side of the room.