It's not about what the cops want.
It's about that a wanted ship (which is at the time under police fire btw) should not be able to report a crime.
It's the same situation when you are sent on a mission to eliminate a pirate lord. You know for sure who he is, you know that he is wanted, and yet you have to wait for the scan result.
It's about you flouting the authority of the local ruling faction. From their perspective you're nothing but a trouble maker they can't fully control. They're not going to give you the benefit of the doubt when they see you barge in and interefere with local matters, flashing your pilots' federation priviledges and arrogance, killing people in their jurisdiction before you've even made them the courtesy of at least asking if that's okay with them.
But the police should not be able to know that you were shooting at an unscanned ship, because that ship, being in fact wanted, should not have been able to report a crime in the first place.
One can imagine a wanted ship doesn't stop broadcasting about being under attack, the police just naturally doesn't come to its help (although from a consistency pov that should lead to a police reponse to come and actually apprehend the criminal).
But ultimately it's about punishing you the player for failing to exercise trigger discipline. That's what is important and that is what needs to stay.