The only justification for shooting at anyone is fear for your life or others. It's really hard to be in fear for your life if you have enough time to shoot someone in the leg or fire a warning shot. In fact, a purposeful shot in the leg means you weren't in fear for your life and were shooting for other reasons. It's a good way to get a long sentence in jail and sued for everything you own.
Police are trained to shoot to stop and to only shoot when their or others lives are in danger. The most predictable way to stop someone is two shots to the center of mass (i.e. the chest) and that's what they are trained to do. It's the easiest to hit with the highest possibility of stopping the threat.
Also one to the haed as well.