A-hem: [stern look at all the silliness posts e]
'foul play' is an odd expression -- how did sport intrude?
Well, could it have transferred over from the expression "fair or foul"? Fair used to be an adjective used to describe a good-looking female, among other things; point being that it has long been associated with positives. So, if any kind of interaction (business deal, a sport) occurred more or less as expected, it could have been described as fair. And I'll bet foul just developed as the "natural" opposite.