[ThomasG]

You may well be right on this one, yet I've never seen any car that would allow temp setting below 16.
And there's good point re- evaporator freezing/ice cover.
Essentially, by thumbs logic, to blow air at ~5-6 deg, coils would have to be in the "minus" temperature range, as the air passing trough cooling element will never achieve temperature as low as that element, and also other way around (easier to check- use home radiator) air passing trough heating element will never achieve temperature as high as that element.

I may have difficulty confirming it with actual numbers of paragraphs or whatever it is called, but I can see logic behind it. Thus, I do believe there is such law.