The question is: "Why is it strange to have a round chambered in a police officer's weapon?" It takes precious seconds that one can't afford to chamber a round when confronted by a "bad guy."
While I don't know what type of holster, if any, the officer had, it is conceivable that the weapon was "cocked and locked," making it more susceptible to an accidental firing.
It takes significantly more than 2 seconds to decide if:
1) Lethal force is required
2) I will offer lethal force
During that time, one can both chamber a round, and present the weapon. My training came as a child from the NRA. We were taught NEVER to carry a loaded gun, until we were ready to shoot. That meant, the bolt open in a rifle, and the barrel exposed in a shotgun (or breach open in a pump shotgun). That way, you simply couldn't misfire, as the gun was rendered into a club. I see no compelling reason why this thought process should be any different with a pistol.