Guns are high value, relatively low weight, so it makes sense to send by air.
Can you imagine how expensive a crate of ammo would be if sent by air?
Has the U.S. ACTUALLY siezed any ammo? When I talked to the guy at Lever Arms, the story I got was that they cannot bring in by ship, as ships are often diverted from Vancouver to Seattle, and would then be subject to confiscation. Also, if a ship carries any Norinco products on this trip, it will be given a hardcore inspection the next time it goes to a US port. Just another way to jerk around the shipper.