You know, this is kind of like how fighters and bombers in Empire Earth and Empires: DOTW have limited flight time. They have a store of fuel, which is depleted over time and, in the case of Bombers, drops instantly to zero when bombs have been dropped. Fighters have infinite ammunition. When it's at zero, the plane returns immediately directly to its airport and lands, where it is immediately 100% refuelled. If it's damaged, it'll have to wait until it's repaired to take off again.
If I had that explained to me before trying either of the games, you'd hate the idea. But having played the game, I found it to be so much better than the ammo system in certain other games that it's amazing.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.