TailGator, since I know you are a veterinarian and are trained on animal and human anatomy, I also know you're aware of the femoral arteries and other blood vessels in the legs. So, I'm pretty sure you weren't so much surprised by the result, as you were trying to point out to others that shooting somebody in the leg can very easily prove fatal to the target.
But I just thought I'd emphasize that: Shooting somebody in the leg is still using deadly force. There is a decent percentage chance that such a shot will in fact be deadly.
Not faulting the clerk - the shot was fully warranted.
I, too, wonder who trained her to shoot for the knee. I wonder if the BG was holding the baby in such a way that a lower extremity shot was the safest option for the shooter, and if she meant she'd been trained to shoot at a body part as far from a potential innocent as possible.
Hopefully, nobody out there is teaching to shoot for the knees, per se.