I think in a situation portrayed in the game the soldiers would be ordered to shoot anything that moves on the other side of the fence. As for why it's added perhaps it gets overrun after a while in game (sometime well after power-water failure) It could posses high level loot (IE lots of non-perishables, ammo and guns) but it would be heavily infested (size of a horde or two) due to all the gunfire that would be happening. The base being overrun could also trigger more zombies roaming about due to the new "silence" coming from the military base