The white countries on the map are “neutrals”, and they are not active or controlled by any player. Similar to pacific 1940’s neutrals: if you attack it, they defend, but other than that they do not move or regenerate or anything.
So, giving poland bombers would have meant they would have some units defending on a 1, since they wouldn’t be doing any attacking or anything at all since they are neutrals and not player controlled. Poland actually has quite a large defense already, considering the small value of the territories there, so changing the units which are in there would have only been aesthetic, and could certainly be nitpicked to death.
There is a mod though, where Poland, Spain, Greece, and Turkey are controlled by AI players, and are able to attack anyone who enters their territory but unable to move beyond their territory. Its a pretty cool mod, and it also uses AA50 rules instead of revised rules, however it is not yet balanced even close to the terms that the “original” map is balanced to.
While historical accuracy is important and always nice to see, Balance is far more important.
This game has been balanced over about 3 years of continuous play by a very large number of players, and right now it sees very small and very few bids.
Part of the consideration for Denmark having slightly higher defense than Belgium I think has to do with the choices apparent in this game. Some games see Germany leaving Denmark alone in order to save themselves from having to defend it against the British. By making Denmark slightly more expensive to take, it makes this choice more obvious.
Beyond that, I would have to ask the map makers, Sieg and EB, the exact reasons for different garrison levels in different neutral territories. I helped to code some of the map, but did not play any part in the balancing of it.