Aircraft are trumping units in this game; they are better than their cost, more flexible than any other unit (since they can be on land or sea). They prevent the game from being like Risk; where only numbers matter and all the units are the same.
This is just like the sub rule, its to represent the overpowering advantage aircraft had and have over all other types of unit during that era. Not sure what objection that raises, except that they are OverPowered. Which is true in real life also, airpower is OP compared to its costs and that’s why they keep using it.