Fixed that for you. Proper pirates won't kill their compliant prey, otherwise they're probably going to be less agreeable to your demands the next time.
It also has to do with the fact that player piracy (in relatice terms) is profitable, whereas NPC most certainly isn't due to their obsession with hauling low value goods.
This is true, but it's not what people are complaining about. I have never seen a thread saying:
"This pirate asked me for 30% of my cargo and wished me a nice day, it's totally unacceptable that these gentlemen are not punished for their civil behaviour and I am going to play solo because if it."
It's more like "just got blown up for no reason", "I travelled all the way to Colonia and got killed", "I lost 400t of cargo (and the killing CMDR can't even pick it up) and a 15 million rebuy", "visiting alien ruins or CGs is impossible without getting killed". And the usual replies are "that's fair play", "open is PvP mode", "don't play open idiot/carebear/crap tier player".
I think it's reasonable that some people have a problem with this type of asymmetric and totally out of game PvP. It's just destroying for the lols, there isn't even RP to it.