Yes, they did air drop ammo but it was probably packed differently than in a crate. They used more specialized drop canisters than a wooden box. Yes, the game name is "ammo canister", but they do appear to be boxes to me in the pictures. Setting the C-47 to a carry capacity of 28 rather than 128 keeps people from air dropping antitank guns and the like, as stated by
thatguy96. Those types of weapons were delivered by gliders, which could carry them and ammo crates as well. Gliders seem to have no trouble loading in the crates, or for that matter, ammo trucks and M3 ammo tracks.
I'm not sure you can edit the OOB to get around this either. The weight of an ammo canister is zero, same as an infantry unit, so the code must restrict loading ammo carrier types(56) on parachute transport types(60). That being the case, if you really need ammo dropped with your troops, I would buy a glider
