Don't get me wrong, I'm not saying don't try it or they are silly ideas, but I playing devil's advocate
"How do they 'now'? "
But artillery is off table, they can't counter aircraft. But you may have meant that anyway?
"I guess sticking an AA gun with the artillery might help? "
And then you have 1 AA units less on table. And many armies have AA restricted to 1 per 1000ap.
"Why would game balance be affected? "
Well, artillery is weak defensively so allowing additional units to target it means it is very likely to not be part of the game after turn 2. Also, I think once lost they count to morale losses (although not to the original count) and so low break point armies will collapse very quickly. Not much of a fun game.
Should we not eschew the traditional view of 'artillery'? Who's to say my artillery are even on the planet for your aircraft to strike? I'd simply declar them off planet in 3 different locations.
If you allow aircraft to CBF (assuming the artillery is planet side) then why not just allow some stands to go off table and destroy the artillery in a commando style raid?
It's can of worm... If your aircrfat can hit my off table artillery, can my artillry hit your aircraft whilst they are grounded off-table? ...A far more effective (and rational)...option
Can my aircraft hit your aircraft whilst they are off-table with an intercept mission?