I am only guessing, but the TAT of 10 deg C indicates a safe margin of the temperature around the aircraft once heated up by friction created during forward motion. Obviously ice can only start to build up when the temperature of the surface around it allows it to. By using the TAT this is a good indication of the actual temperature of the aircraft outside body.
Why the SAT is used is probably along the same line of reasoning. You need a sufficient amount of moisture to create the formation of ice. And in temperatures of -40 deg C and below, the actual watervapour contents per volume is minute, therefore the risk of ice building up is almost nothing.
[This message has been edited by Skyjob (edited 19 January 2001).]