That's a good question. I was a user of a website some years back, and soon became staff (Moderator, and then Administrator). We were more community staff, as the real Administrator/Webmaster wasn't around often. He just set the forums up and did the technical work for them, and the company had another IT guy who dealt with the servers dealing with the business.
We upgraded to vBulletin forums after having phpBB, and a big vBulletin fan was a member and somehow caught it was pirated, fault of the Administrator/Webmaster. I'm not sure if it was why, or if there was also other issues (I think there were), but he left/was fired, and they sort of let me take control of the website. They offered payment, but I did it willingly, until years later hosting issues/lack of communication led to fallout and the site was deleted (hosting sites fault).
Anyway, that was a similar thing, only it was the forum software, not the operating system, that was illegal. I want to say the one directly responsible would be at literal fault, but in court, with bigger parties, who knows.