Pirating on Discovery, as some people have pointed out, isn't "real" pirating. It's extortion and protection rackets. Since the NovaPG ID doesn't specifically say that they're allowed to pirate trading vessels (as I believe other pirate IDs say), then they wouldn't be allowed to tell traders "2mil or die". Asking for cargo is, as Unseelie points out, is more like what the lawful factions get to confiscate contraband. I don't know what kind of cargo the NovaPG would be asking for.
So the NovaPG is allowed to blow up people, traders, whomever they want (within RP), and they can demand cargo from traders (as long as they aren't in big ships), but they can't ask for extortion money. Weird.