The use of AR-15 rifles in law enforcement is about as common as radios and gun belts. Although some (mine included) carry them loaded and chambered, many agencies choose to carry their rifles unchambered, or "cruiser ready" as some call it. This means there is a loaded magazine in the receiver but no round in the chamber. The operator would need to pull the charging handle to the rear to chamber a round upon deployment. This is a safety practice due to the AR-15's floating firing pin.
A few of the downsides to carrying an unchambered rifle are slower deployment time and the uncertainty of the condition of your weapon. Historically, chamber plugs or chamber flags, small colored plastic inserts placed into the chamber, were used to provide a visual confirmation the rifle's chamber was empty. Unfortunately these plastic plugs were less than robust and had to be manually removed from the rifle.
