Hmmm.... where to start...
Okay first off, a UNIT of AA does not mean one piece.Hence there is no "one Anti AA Gun" to be explained.
Next.Almost every unit in the game is considered to have some anti-air capability.Something like shoulder launched stingers .Sooo its not a question of "some guys with AK's".
Now, despite the wrong assumptions,you still have a point.
First ,It has been mentioned there may be a bug with garrison units performing to well against air units.Being looked into, and the devs have a good record with fixing things.
Second, the power of anti-air has been discussed for a very long time as possibly being to powerful itself.Debate is still on going.Feel free to find a thread and join in.
Now, a suggestion.There really is no need to expose you air units to alot of enemy fire.Missiles exist in the game, and their designs can be traded for, allowing you to build them and than arm your units with them.
Designs for aircraft can also be traded for and than built, so even if your current selection of air units is sub par, you have a route by which to attain units of more use/worth.
Modern airforces swear by those modern missile gadgets and some of them new fangled flying machines