A ticket is something you do not buy to attend a convention. A fan pays for a membership in an sf convention to help cover its costs, because the fannish way is to be an active participant in the sf community, not a passive consumer of entertainment. (A ticket gets you into a show as audience; cons are not shows and their members are not audience.)
"Tickets? We don't need no stinking tickets!" a fannish catchphrase, reflects this philosophy, although fans' use of this misquote from the 1927 novel and subsequent, 1948, film, The Treasure of the Sierra Madre, stems from an unappreciated effort to control masquerade seating at a Worldcon.