It's not really a penalty. As someone mentioned, if you have even just a 1 era technological advantage, those 1 hp dings aren't going to stop you unless you're playing increadibly cheap or simply can't war.
But I think overpowering unit kills defender with no damage is highly unbalanced.
In fact, I appreciate the fact that bombing missions forces bombers to take damage. Other iterations of Civ did not have this penalty, only that your planes could be shot down outright if there was sufficient Anti-air in the stack.
It made for really really overpowering airpower bombardment under human control and certainly is a situation similar to how ships can kill infinite number of embarked units right now.
Granted it is still possible to have your superstack of bombers parked in the city achieve something similar, but taking damage has tended to slow this down.
When the debate is reality vs. elegant simple solutions to game design, game design should always win.
No... no it shouldn't. It's just a glaring "What?" moment and prime snark bait.
If your units are horribly outteched by the other side it makes perfect sense for you to be losing, badly. This is why antiaircraft weapons exist - you have to make them or make units with that capacity or accept the fact that you're going to get Death From Above. This is why research is important.
And once again. Supplies makes no sense.
If it's fuel, then why doesn't the tank loose health for being outside of friendly territory, or on the other side of the planet? Why is it that the tank apperently only uses its engine in battle?
That's true but the tanks would have to spend fuel and ammunition, reducing their combat effectiveness by a small increment, say about one points worth!
If it's ammo, why do enemy ranged attacks do a minimum of 1 damage? An archer should not even be able to scratch a tank; the tank doesn't seem to be shooting back.
In addition, usually vehicles (like bombers, for instance) don't explode when they run out of ammo. Health is not the same thing as supplies. Supplies would be an interesting independant addition, it shouldn't be the same as the health bar.
It's an arbitrary penalty that should be removed, and was probably due to a rounding error (damage calc rounds up even infinitessimal decimals to 1) in the first place..