Why traders don't pay?

[The Savage]

This goes both ways, what you described is the exception. Most of the pirates are like:
:: Give me 20 million and your cargo or die.
Then one starts and tries negotiating and next thing you know is:
:: You didn't fulfill my demand - opening fire

Requested credits must equal the cargo's value. Otherwise - sanction. I mean, you can still 'tax' an empty transport, but it shouldn't surpass 1 million credits in such a case. I know this is not a poor player, if he/she can afford transport with more than 2000 cargo, but on the other hand he/she hauls nothing.

But then, I've realized that my piracy recently turns into escort duties due to the cargo and shipment destination, so I always take advantage of that fact and simply roleplay with the smuggler, gaining even more money from escorting them rather than extorting them.