Actually, some pirates don't. Me, for instance. Varyag probably expects the traders to be in a Whale and the pirates to be in a gunboat.
Also there is the not expressly stated prejudice that traders come without escorts.
But it can also be the other way around.
An Advanced Train has 6 Trader turrets. The IND are flying cap 8 armored pirate Trains which will fire 7 at you. And if that sort of firepower was allowed to enter the system again after they were destroyed and fire on you again after you make your demand ... eh. Sorry. No.
I've given you the counterexample but regardless of anything above, the rules are rules and interpreting them wrong is not a right way to go about it.
You couldn't find that exception. That is completely right. There is none.
However there CAN be a gentleman's agreement ... the pirate can let you trade through the system and try to catch you again. But you can't force it, really.
Because there are and have been and will be traders that
1. don't say a word
2. never pay
3. shoot at you as much as they can
And because they don't drop a lot of cargo when you blow them up and even repairing the cheapest combat-worthy ship costs hundreds of thousands.
Personally I can tell you this: If you get blown up and enter the system I'm in again ... and open fire on me again and I don't like it ... I'll blow you up for the second time and you get a warning. Come in for the third time and you're not just risking the cargo but also all the cash and guns.
I have started a similar thread before, please read: