They've been asked by the promoter and agreed to do this as a fan favor, signing a contract that prevents them from ever doing it again/doing it somewhere else, would be very stupid on their part. If they just keep quiet about it for a few weeks works for the festival sales well enough.
Remember this is what the promoter said, not the band, he wants to sell out the festival. He also didn't use words like world exclusive, or once in a life time event blablabla.