Continuity mistake: As Hawkeye talks to the wounded man on the chopper, the speed of the rotor blades changes several times in the scene. Looking at Hawkeye, the blades are moving fast; in the reflection, much slower.
Continuity mistake: When Hawkeye and BJ unload the chopper, the dog, Corp. Cupcake, is lying on its paws in the long shots and on its side in the closeups. (00:03:35)
Answer: Even an Army field hospital would have SOME weapons for self-defense if the enemy attacked. What's meant is that it's not a combat outfit, and therefore wouldn't have enough weapons to actually attack anyone. (It's also mentioned a few times that the Officer of the Watch is supposed to carry a pistol at all times, but Hawkeye refuses to do it when he's in charge and Colonels Blake and Potter don't enforce the rule).
Captain Defenestrator