Because there's not such a rule, punker, and never were.
Killing someone who's trying to escape is perfectly valid. That's why quickdocking exists, to help people flee from combat.
The rule was that it wasn't allowed to kill someone who was undocking, because he couldn't even see the enemy before he was dead.
If you are fighting someone, you know he's gonna shoot you until you die, or you flee.
IN RP many things could happen, you cannot affirme that Cadiz would have blown him up. IN RP, cadiz could be blown to pieces as well by a hessian fleet, with you inside, or not.
But, to make it clear, there's not such a rule preventing people from getting shot down while DOCKING.
There was one for UNDOCKING, and it was removed as there's a period of time while you cannot get any damage.