[Marcello]

"I'm guessing the reasoning behind this is because this is a game, and a SP artillery piece that requires an ammo carrier to essentially be attached to it at all times would be sort of worthless in game terms."

Why?
After all what would happen in reality? It would have ammo carriers/dumps/whatever nearby. And since if I am not mistaken ammo loadout is one of the main variables in determining artillery units prices, I suspect a two rounds load would probably make the vehicle cheap enough to spare enough points to buy an ammo truck. I will test it to see if it is the case.