Quite all arguments and stories are known here. So don't expect too much.
Flying without shields is risky and was ever. You need to take the math if changing schields for cargo will
bring you more profit. Means if your summed up profits for the delta of cargo will weigh out your rebuys.
This depends strongly on choosen route and does not fit for everyone.
Probably, he and others like SDC prey on shieldless people. They don't care if you actually trade and usually go for easier targets if they see you having a shield equipped as you aren't in violation of the "have a shield when trading in Open" law and would rather farm the tears of those who fly shieldless and have to be taught the hard way that a few extra credits per hour/day isn't worth making yourself a helpless target that dies in seconds... whereas if you have a shield you can submit to an interdiction, select a system to high wake, boost at your attacker, charge and jump to said system, and be alive just fine... but if you don't have shields then that first pass while trying to escape will instead be a lethal one. Heck flying shieldless means that even if you try to combat log on your attacker you're probably already dead before the server takes your status as offline, so next login will be you greeted with the rebuy screen.
As much as this looks like ganking a noob, or preying on the weak or whatever, the target WAS pledged to an enemy power and was thus fair game for execution without terms.
That said, I am eagerly awaiting the new karma/C&P elements to come in with 2.4
If I am seen with no shields then my boat is heavy with as much armour and resistance as I can fit to it. That and teeth to shoot back with! But that OP was not INSTA-DEATH!