Gregg did make sense there.
There's really many considerations, especially since it's more likely to be a open-for-all type of shoot.
The penalty will be really unfair to novices and amateurs.
But taking it in a more professional or competitive perspective, I still find the penalty pretty legit. Not that a penalty can cause anyone to do less harm to animals, but it's just the "guilt-factor" for the game. Makes everyone more focus on landing a kill shot I feel.
And I read a little on 3D target shooting, seems like most clubs uses 11~12 for small circles, 10 for heart, 8 for lungs, 5 for body, 0 for antlers and hoofs.