NPCs don't want cargo and don't stop for cargo anymore - if they interdict you (and not system security) they will try to kill you.
It also does not matter what cargo you carry if any at all. They also don't pick on a ship type, NPCs will pull any ship, not just traders.
Well see, now we have a positive for human pirates.

I'm glad we uncovered that stone.
AI pirate: "Arrrrrrrrrrrrrrr! Prepare to die! Gimme your cargo!"
Human player: "No! Don't! Here! Take it!"
AI pirate: "AHahahahahahahhaha! ARRRRRRRRRRRRR!" *shoots player with lasers* "Heating up!"
*Human player dies*
-vs-
Human pirate: "Please remain calm while you are scanned."
Human player: "Okay."
Human pirate: "ARRRRRRRRRRRRRRR! I spy Imperial Slaves. The tax be thirty tonnes! Abandon thirty and ye may go free!"
Human player: "But Mister Pirate, Sir, I... I'm down on my luck. See, my girlfriend, she dumped me and left me with her smelly dogs--ran off for some stud named Rico and his fancy Ford Escort! He's buff, and those chrome hub--"
Human pirate: "Yeah, whatever. Gimme the cashhhhh!"
Human player: "Please! I'll do anything!"
Human pirate: "Anything?"
Human player: "Anything..."
Human pirate: "Hmm. Okay. Tell ye what I'm gonna do. I want ye to dance fer me. Ye dance and drop me twenty and ye can go!"
Human player: "Dance. Oh. Well twenty is reasonable..." *drops cargo and starts to dance with his thrusters*
Human pirate: "Yeahhhhh. I like that. Now do a little twirl."
Human player: "Like this?"
Human pirate: "Yeah! Perfect! Now hold it! Hold it! ARRRRR! Thank ye kindly, 'ave a nice night."
*Human player flys off into the stars, a little weirded out at the whole encounter but no worse for wear, ship still intact and continues about his business minus a few tonnes of cargo.*