Cops are intentionally on a different playing field to criminals. They are the whole gameplay counter to being a criminal. Makes no sense to have them subject to the same risks as criminals. As a criminal, you should be wanting to avoid police interaction, because the consequences for if not will be drastic.
They can still be PK'd, but unlike criminals most of the situations they are in they are simply doing their job. It is not a criminals job to be arresting people or conducting lawful raids, why should they be punished for that? And why should it be a frequent occurrence?
The main argument mentioned in this thread is about cops not "valuing" their life. If you make the decision to try and shoot and cop during being detained and get blasted, that's nobody's fault but yours. You made that decision, not them. Maybe don't be so quick to resort to violence, which I will repeat again is YOUR decision to do that.
Think objectively for a second and not what selfishly benefits your roleplay as a criminal, you'll immediately realise why things are structured the way they are.
If cops were subject to the same consequences for dying in certain situations; the server would immediately become stale because criminals have no risk and would be able to wipe the floor. No.