I think it might also have to do with the additional rules/exceptions not found in other AA games such as CAPS, airfields, naval bases, Japan’s first round attack on US forces (defending at 1).
According to the user manual Allied units (both Commonwealth and Us forces,…not only Us forces) may defend only with a roll of 1. Of course, any attacks made in Chinese territory are defended by rolling at full strength.
It would definitely unbalance the game to limit builds to the IPC value of the territory. The Allies have a hard enough time as it is preventing Japan from winning the game (especially to a 22 VP target). If you handicap them further by not allowing the US to build more than 2 units per turn in Hawaii, the game is lost for the Allies.
