My bad, I don't know why I thought it was the other way around.
What are cops supposed to do, then, if they see someone raiding without an advert? It's a much more common thing than cops RDMing and saying that the victim was raiding a base. In fact, I've never personally dealt with or heard of such a situation. If the cop arrests the player instead of killing them, the situation/resolution is the same. No proof either way, but it's an arrest rather than a kill.
Sure, the cop should report the player raiding without advert. But do you see cops checking the chat to see if someone adverted raid before taking action?
Either way, the fact that there is no rule stating that cops can't outright kill those committing illegal activities still stands. Civilians can counter and kill, and cops can too(with the rules written how they are currently).
Again, I think cops arresting more players rather than killing would be perfectly acceptable. I'm not out to play CP just because I want to kill people, I would play thief and raid random bases or packed gun shops to do that. I'm just saying, the staff enforces a nonexistent rule.