Well, its actually kind a nuanced rule, because it creates a risk when the Axis take something over, that the allies can liberate that. It especially applies to the mahgreb, Syria, etc–if the Axis ignore those lands, they can never be liberated or host allied bases. If the Axis ignore them, they often become allied landing spots, so that’s rich play as well.
FiC is a good example too, because its an easy take for Japan that often gets taken back by another Ally—so its cadgey play by Japan to leave it alone if they cant hold it.
If you mean its not realistic, its worse than that; the French were supposed to be an Ally, at least in the mythology of the War, but did much to resist/impede the Allies…so after the fall of France it would be just as or more accurate to depict the Vichy French as Axis…as in BM…which you could very easily HR.