Why isn't the Presidential 747 "Air Force One Heavy"?
Practically speaking, the 'heavy' designator ('super' for A380 and An225) is to help enforce separation requirements due to wake turbulence. In case of AF1, it is not as if other aircraft are going to be allowed near it, so the designator is redundant.

Regulation wise, FAA JO 7110.65T Section 4. Radio and Interphone Communications, specifically states that the 'heavy' designator should not be used:


  
  When in radio communications with “Air Force One” or “Air Force Two,” do not add the heavy designator to the call sign. State only the call sign “Air Force One/Two” regardless of the type aircraft.