It's always... "He went for my gun so I grabbed it instead and shot him".
Cop holsters have built in security features. It is very hard to remove them except for at an exact angle and you may need to move the gun a certain way in order to retrieve it (wiggle it left and right for example). These are known as retention holsters.
http://www.gundigest.com/concealed-c...ster-retention
These are clearly a very good idea, but your average bad guy probably doesn't even know about them. It is very hard for someone other than the owner to remove the firearm. As it should be.
But "He went for my gun, so I shot him" seems to be the usual line of defense for a cop shooting someone. When we already have mechanisms that would make such an act very difficult at the best of times.