The cavalry killing those horses, should be viewed as a testment to their toughness and adilty to be used as cavalry mounts. It was common pactice for the tribes' ponies to be taken into army control.. This hurt their pribe and removed their strongest advanage the trides had over the army. The thought of a herd of Appas getting into the hands of Sioux warriors, was enough to make any trooper or field serving officer very nervous. It could be compare to a M1A2 Abrams into the hands of Sadam's Army in 2001. I am miltary history and cavalry bluff.
. I am not saying it was the right choice, but I thought why it was made might help.