"The show must go on" is not just a cliche in the entertainment business, it is a central part of the contracts between venues and talent. If the Avalon Foundation were to cancel a performance the act still is owed for the performance, often tens of thousands of dollars. Consequently, the Avalon Foundation stages all performances so long as the performer is present and able to play. Likewise, contracts frequently include provisions which mean that as soon as a ticket is sold a large portion of the ticket price belongs to the performer. This is why there are not refunds or exchanges. It is often not the Foundation's money to give back.
When the performer cancels the Avalon Foundation makes every effort to reschedule in a timely way. In the event that a performance is permanently canceled or the new date is not convienent to ticket holders full refunds are available.