Because pulling ammo from thin air makes no sense and people will just stun lock the boss with crits until he is dead. So by taking ammo from reserve you might run out and have to either use a synthesis or find some more special ammo if that's what you want to use. You can still fire without reloading you'll just have to pay attention to how much ammo you have now. Otherwise black hammer was basically an exotic. I actually wouldn't mind it being an exotic with the white nail perk.