As far as I'm aware, I lack zero knowledge in IT, but I'll venture to say that the U.S. (and any other) military spends the majority of their time training to dispatch an enemy from as far away as possible. Which, in an era where a sniper can hit a target one half a mile away, makes perfect sense to me. No one is suggesting IT can stop bullets are they? If they are then.....I call B.S.
I would say that Rob's discussion above is probably the reason why, but the benefits of the training would be useful, namely carrying larger loads with less fatigue, increased postural stability should allow for more accuracy (anyone who has seen the surprising results of Akuzawa shooting with minimal experience can attest to that).