Here's the problem:
Bombers are very expensive pieces of hardware with highly trained air and ground crews. Acks are mass-produced guns with smaller and less highly trained crews.
In AH, you can kill an ack and it won't come back for thirty minutes. If you kill a buff, it can come back immediately. So, if you allow any gun on the buff to work on the ground, you have the absurd consequence that it's profitable for some idiot to just keep respawning as an ack platform. That's bad for gameplay. It gets worse when they decide to strafe troops.
This is why guns were disabled on the ground int he first place. People would line up in continuously respawning b17s, shooting at anything that came by.
If you want to talk realism, there's no way IRL you'd tell a bomber crew to run into their plane and get off the ground while the field was under attack.
So, if the field's under attack, don't go up in a buff. That's stupid. (well, if ALL your fields are under attack, that's a different matter).