In any case, Grand Final tickets are on sale, so the fans of the winner of Friday's prelim get the first crack at remaining tickets. Kinda the same thing.
On that point you are in error. You'll find the NRL don't put the final tickets allocation for the Grand final on sale until both teams are known who will be playing, and the ticket holders of both clubs get first crack at them before they go on sale to the general public. The two competing clubs season ticketholders get access at exactly the same time.