And that only helps your power if its under circumstances that actually makes the numbers move. For example, if CZs can affect PP (or CZ missions to kill X number of enemies), then killing an opponent will count as 1 towards the number of kills that needs to be made, but even then, they get to keep their kills made already, and you can kill NPCs a hell of a lot quicker.
If the opposing PP player is attacked when they aren't even doing anything PP related, then the attacker has effectively wasted their time, at least in terms of PP.
If the opposing PP player is doing a mission that would affect the numbers, then you will slow them down a bit, and its possible they will lose the mission (depending on the mission). You don't make your numbers go up, but you do delay the oppositions numbers going up. If they have a mission that can be continued after death, then they just restart and go again, perhaps this time avoiding those who attacked them.
But those hanging around simply looking for PvP kills will never make their own numbers go up. At best just delay an opponent.
But then, if we consider PvPers going after those opposing them, they are happy not helping their own power's numbers go up, just delaying/stopping others, that would be reasonable PP gameplay, a worthy goal, if that's how they want to spend their time. But then who is actually making the numbers go up on your side? Right, those who are doing the same as the people you are attacking. So, don't the people on your side also need defending against opposing PvPers? Should you really be spending your time looking for opposing PPers or would it be better escorting those on your side to protect them?
A lot of these discussions about PvP are framed in the context of gankboat(s) getting their gank on against opposing players flying non-combat ships.
Never (? or almost never) do we see PvPers talking about escorting their own non-combat ships who are trying to help their power.