So there are very different situations -
Imagine attacking Iraq with the UK early in the game with a BB and cruiser. If those both hit, 2 of the 3 infantry would be eliminated and not be able to fight. What, your battleships in WWII can fire from the Persian gulf all the way inland to Baghdad and annihilate 2/3 of the army?
Or you have a situation where you’re suiciding 1 ground unit into a big stack, with battleship support. In this case it doesn’t matter if the bombardment casualties can fire back or not, because there is a virtually 0% chance that the attacker will fire twice.
With scrambles, kamikazes, combat in zones (to clear a transport, sub, or destroyer, or whatever), there isn’t much bombardment in the game anyway, so changing the rule wouldn’t make much difference, but could give you extreme situations which may be undesirable, like the Iraq example