How? Not all officers would agree with that statement. Every vehicle has unique qualities that some will prefer over the other. Some police would feel cool being squashed into a Camero all day long, others a spacious SUV and yet others something in between.
We aren't talking "movies" here, we are talking real life, one in which a police officer turns on his blue lights and pulls someone over, the car pulls over, the police officer has plenty of room on the HD to maneuver with its low center of gravity and generally spacious bike, well equipped to handle rain and high winds in an emergency to get back to the station.
Need to make sure we dont have visions of motorcycles racing through streets and interstates at daredevil high speeds chasing suspects. It doesnt happen, the officers life is more important in training 101.
Seeing a Harley and being pulled over is more imposing then being pulled over by something looking more like a moped, throw in all day comfort too and safety being seen by other vehicles.
I have nothing against BMW but they are two different vehicles, much like the 4 wheeled vehicles in a police department are different. How a BMW motorcycle will help a police officer perform his job more effectively is kind of out there without a reason.