Made me quite happy, because I was reminded of all the Americans who vehemently insist that shoot to kill is the only option ever, even if the person only has a knife.
The way it was explained to me when I asked the very same question (I don’t necessarily agree, just offering this officer’s viewpoint): discharging a firearm is a deadly act, which always carries with it potentially deadly consequences. It should only be done to save a life, whether yours or someone else’s. The problem is that if you shoot without the intent to kill, you’re saying that you weren’t really in fear for your life, you were just stopping a lesser threat.
It's that center-body-mass is the largest target to shoot at, and therefore the safest to shoot at in a public area. Safest for everyone except the one being shot at, that is.
God, does that make sense? It's hard to explain that. You want the largest part of the bad-guy to catch the bullet, to make sure the people around him don't. Therefore, shooting at the (relatively) stable center of his chest is a heck of a lot easier and safer than shooting at his moving arms or legs.
And you may need to hit center mass a few times before they actually stop, so don't get cute trying to shoot off fingers when innocent lives are on the line. Which is the only time it is acceptable to shoot anyone, ever.
The reason people shot center body is that it is largest part of the body and it contains the most vital organs. Thus a shot there is most likely to be fatal and to keep that person from continuing to be a viable threat.
Given all of that it is easy to see that shooting an armed and dangerous individual in the leg is not a good way of dealing with them. It makes for a great feel good story but in reality it is not a good tactic.
In a life or death situation (which is the only time it is acceptable to shoot a person), nobody has time to make trick shots. If the target deserves to die, aim center mass. If not, don't shoot at all.
Imagine if the cops tried to "shoot to non-fatally disable" someone who didn't deserve to die, but killed him anyway? That would be murder. You don't shoot at anyone if killing them isn't justified.
Then if you miss and shoot the gun out of their hand, or whatever the movies have told you is the easy, humane thing to do, well, that's a bonus. You can breathe easy knowing you didn't end up killing anyone.