I don't see any problem in here. Why? Traders usually log(ged) off when they dock(ed) - or leave the system. If that's the serious issue, take down the PVP Death Cooldown to just 1 hour. Problem solved.
Take into consideration that docking didn't end the engagement because it didn't end with PVP Death (I asked an Admin long time ago about that, out of curiosity) and pirate was still allowed to continue to shoot at the trader. I think that's why rule got changed in the first place in order to disallow pirate trolls to down trader without any further word just around the proverbial corner.
If that doesn't convince you, the same trader could still be taxed by the very same guy in the same system, following the old rule exception. And that's why I said that "traders usually log off when they dock".
TL;DR We simplfied the rule and took down the gray area which the exception created by accident.
