There is a huge difference between an infantry and an AA. The difference is, the infantry can be destroyed.
Case: Japanese air power wishes to clear a path for the German advance. Sorry, don’t even bother, because an AA stands in the way and will block a blitz and cannot be destroyed. The AA’s intended purpose of shooting aircraft is NOT used because there is no purpose in attacking the space, so the combat never takes place. Second, the unintended use of the AA IS used to block the path of blitzing tanks and Russia loses no units and pays nothing for this strategy.
This is called gaming the system; when unintended uses of the game are employed for purposes other than the intended.
Someone mentioned the idea of capturing the gun….except a solid Russian strategy is to not bother defending both zones that protect Russia (Belorussia and Eastern Ukraine). A single mass of units exist in one zone (along with an AA), while only the AA exists in the other. If the Germans try to capture the gun, they will be taken by the mass in the other zone.
The use of the AA to block the blitz allows the Russians to protect both zones effectively while only occupying 1 zone and risking no units.