On the .50 :
While I agree its odd, I can see the reason they designate it as not-for-use-against-infantry.
The reasoning is the same as follows : This isn't a well known fact, but standard NATO ammo is _specifically_ designed to wound and not to kill. That is, the rounds are full-metal-jacketed - designed to pierce completely _without_ expending all the energy on its target. Hence creating a wounded soldier almost as inert as a dead one. This is in contrast to say, cheapo hunting ammo, which is specifically designed to mushroom on impact with a target to expend the maximum amount of energy on the target (stopping inside) and making the biggest hole. (I always get kindof amused when people say - oOOoOOooo eeeeevil "military" ammo. If I had a choice, I'd rather get hit with one of those evil military rounds rather than the somewhat PC hunting ammo).
Anyway, that gives you the flavor. The idea is to take out the enemy, not butcher him. Granted this kind of flies in the face of the reality of artillery, grenade, etc, etc. But no one has ever claimed that war makes sense.
Thus ends dave's firearms history lesson of the day.