there is quite a distinct GAME reason why this was “unsplit”.
When there is just 1 territory up there,
If Japan takes Alaska, tanks from the US can cross direct to attack.
If the US has units in West Canada, it can make Japan’s invasion position insecure.
When there is only 1 west Canada;
Japan cant block the southern one and hide in the upper square and Alaska or land its planes up there in force (10+).
When there are 2 squares of Western Canada, it can make the Japanese position very difficult to dislodge, because on the second turn of their invasion, they can put forces both on Alaska and in that upper, northern square where they can’t be attacked amphibiously. Also, any tanks on Alaska can drive onto the other mapboard unblockably because it is much harder for the US to block/strike both squares, esp. just after the USA got invaded. At that point, Germany lands on top of the hidden, protected square with its airforce and now they are unstoppable and can occupy all of America’s attention and $$ (Operation Hollywood).
I think this was done to keep America from having such a hard time of pushing Japan out of that area and not giving Japan an unthreatened backfield that they can hide in to prevent an American counterattack.
Most Japan invasions of USA begin by splitting your forces both land and sea to block all those squares that the USA wants to cross through to strike your forces, then flying your airforce over to protect your beachhead. Larry may have seen how abusive it is to give Japan an LZ that the US has such problems hitting.