I kind of see what people are saying about they are pirates so they steal, but there has been absolutely no hint at the ability to steal ships ever in this game, so why can npc pirates steal?
There is absolutely no hint in the game that you can land your ship, get out, go knock over a liquor store on the station somewhere, run back to your ship and make a get-away... but within the world that this game (partially) simulates you could do just that.
I'm sure there is a story for how pirates get ahold of ships that require faction. Maybe they used to have rank in the faction, they got a ship, then they lost their rank when they turned to a life of crime but didn't loose their ship. Maybe there's a black market for crappy fed dropships because they're the Atarii 2600 ET cartridge of the Elite universe.
Either way, you don't know that it doesn't make sense because it would be a story issue, not a game mechanics issue.
In my mind the two things that NPCs do that most destroys immersion:
1> They literally sit and spin while you kill them to death until they are dead. Deffo some kind of bug in the AI there.
2> Even though they were like, only 5 km away while I was slaughtering 5 other ships from their EXACT SAME FACTION, they scan my cargo and if I don't have anything they just turn their back on me while I follow them at 2km until my KWS is done and then proceed to practice proctology all over their 100 million space buck Python. Really when you kill a zillion pirates from a single faction you should end up on their number 1 most wanted list and get attacked by them at every juncture. The way it is now smacks a little too much of bunny farming in a theme park mmo for my taste.