Leagally, (yes, I am a lawyer) having a ticket gives you no right of entry. It may be revoked at any time for any reason. For example, lets say you bought tickets to the best hypothetical concert of all time, a Pink Floyd reunion concert. You score front row tickets and a pass to hang-out with the band after the concert.
You bought two tickets for $5000 a full year in advance. You get to the concert, find your seats, are sufficiently stoned and ready for the best experience of your life. However, five minutes before the concert begins, the manager of the facility taps you on the shoulder and tell you that you have to leave because President Bush showed up and they are giving the preseident and the first lady your seats.
You are SOL!