It's FD's definition of legitimacy, and defined profession.
Except FD made it clear that trading, exploring, smuggling, bounty hunting, piracy, mining are all legitimate profession and game play.
It's not rules, it's a general atmosphere of reciprocity and acquiescence of game play encounter legitimized by FD themselves.
Yup. It's not rules. And nobody is obliged to take any notice of atmospheric phenomena (assuming they know anything about this 'atmosphere of reciprocity' in the first place - I note your selective quotation fails to address this significant point). If they don't like pirate pew-pew, they can exit the game via the 'legitimized' menu option.