California BPC 22507: "The ticket price of any event which is canceled, postponed, or rescheduled shall be fully refunded to the purchaser by the ticket seller upon request. Any local jurisdiction may require a ticket seller to provide a bond of not more than fifty thousand dollars ($50,000) to provide for any refunds that may be required by this section." (Added by Stats. 1986, Ch. 378, Sec. 1.)
Ticketmaster is based in California. IANAL and don't know how this law might apply to events that are held elsewhere.