2) Per [1] the surface area of your torso is about double that of one leg (that is, anterior torso is 18%, an individual leg anterior is 9%), so it's far more likely to hit if you aim for the torso. Even if you aim for a torso and miss, you might hit a limb or head - it's a lot less likely to miss a limb and hit something else.
3) The research behind the Tueller anti-knife self defence drill found an attacker with a knife could cover 21 ft / 6.4 metres in about 1.5 seconds. To stop the attack, you have to be able to shoot them before they can close to melee range - you must aim for the largest possible target to have any hope of success.
I am not commenting on US police practices generally, but specifically that the idea you can shoot to wound is neither responsible nor practical.