There must be a certain amount of historical accuracy in order for the game to “feel like” the subject matter. Axis & Allies has always dealt with this accuracy at a macro level, striving for “feel” rather than simulation, and thus not dwelling on minutiae. It’s a fine line to walk, but some historical realities must be observed in order to maintain the ambiance. In this case, the restriction presents the feeling of threat without overly burdening the Japan player, as forcing avoidance of all USA territories would.
I won’t pretend that there aren’t game play reasons why this restriction is in place. If there weren’t, why burden the game with it? However, any such rule must be grounded in historical events and realities in order to not come off as “gamey” and ruin the feel of the experience.
All of that being said, the USA did rather famously (infamously?) allow the IJN to get within striking distance of Hawaii, as well as several other of its Pacific possessions, without raising much of a fuss until it was too late. I doubt the same would have been true if the mainland had been so threatened (my original post did make this distinction). In game terms, the “threat zone” of the mainland extends two sea zones out. Since the Hawaiian sea zone is outside of that radius, and since Hawaii could just as easily be attacked from Japanese-held territory (Marshall Islands), there was little point in game terms of excluding Japan from that sea zone.