Perhaps the guy lived out that way, far from where the con was being held, and this was his first chance all day to sit down and enjoy himself in pleasant surroundings, away from all the crowds. Perhaps he wasn't Australian at all, but was visiting from the U.S. or England or some other civilized country where people commonly wear suits. Perhaps he likes being dressed up, it never occured to him to care whether random strangers in a restaurant approved of his attire.
I'm glad you enjoyed your evening out, Ozzie, and I hope the gentleman you saw did, as well.