There was a similar controversy during the Civil War in which an artillery barrage ordered by General William T. Sherman killed Confederate General Leonidas Polk (in fact, Polk was cut in half by a shell). Sherman goes to deliberate lengths in his memoir to say that this was not intentional but simply meant to scatter the cluster of officers who were observing his troops. As I think your question implies, this is somewhat strange considering the strategic advantages inherent in killing the leader of your opposing troops. You would think he would claim credit for it!

I would venture to say that aside from the general notions of honor and military protocol prevalent among military elite at that time, another part of it is that leaderless troops aren't exactly good for anyone, even for their opponents. The chaos and the disorder that go along with such an event do not necessarily make for decisive victory, can lead to slaughter and uncontrollable troops, nor always settle the conflict as it needs to be settled.

I wish the OP would supply any other sources for this quote. we had this very issue on this forum probably about 2 months ago and turns out it was mentioned in a book published about 1860 but since then no one seems to have repeated this story. I think it likely to be apocryphal and it could be a story that is meant to emphasize Wellingtons famous admonition to the artillery to not shoot at other artillery. He was very strict about this and at one pt during the battle he thought one of his battery commanders was doing this and was furious, but it was misunderstanding.