Living near a police station, I have to assume these officers driving have X-ray vision and can be 100% sure that they don't need to indicate. Just as many officers also have no qualms with ignoring a no-right-turn at a set of lights further up the road from the station!
I saw a police car flashed by a speed camera on the M62 at night. It obviously wasn't legitimately on police business, as they immediately slowed down to the speed limit afterwards! (Or, as they might claim, they coincidentally were told to stand down at that moment).
How can you know your surroundings when you don't know what anyone else is doing? (because no-one is indicating).
The point is that, if there is anyone to benefit from your signal, then you would indicate. If nobody might need to know, then you don't need to, because it serves no purpose.
If you are just indicating every time, because you can't be bothered to look around to see if anyone needs the signal, then you could actually be paying more attention to what is going on around you.
Looking for people that might benefit from your signal, and looking to be aware of possible hazards, are not completely separate things!
If there is, or might be, somebody to see your signal then you should know where they are (or be aware of where they might be 'hiding' out of view). Hence, it should not be beneficial to just indicate automatically.