When you get attacked/shot I think the natural reaction is to clutch/tighten your muscles rather than go limp and drop things. Training seems to ignore normal human reactions.
"Again, they are trained to shoot until the person is no longer a threat. He never dropped the knife."
Great point with a huge flaw in you reasoning. HE NEVER DROPPED THE KNIFE. Then why did the other cop have to stop him when he tried to reload? It had only been a few seconds and HE WAS STILL HOLDING THE KNIFE and he was STILL a THREAT according to you. The officer should have been allowed to reload and shoot him 16 more times as he was still holding the knife and was a threat. Correct?
Nothing changed between bullet 3 and 16 according to your sources -- he was still a threat -- so why didn't they keep shooting after that?