As I understand it, celebraties have little or no say when it come to promotions of projects they do such as movies and shows they are involved in. It is part of their contract to promote such projects and make pre-determined appearances. I use to handle an account for David Copperfield, and worked with his financial advisor and business manager. They were telling me that when he signed a deal with say HBO to do a show, part of his contract is to appear in certain venues and shows to promote his special.
So if I were a celebraty, I think that my appearances will be predetermined by managers, publicist and contracts. I am sure these celebs find out about it as a part of their schedule, rather than their own planning. Example, Tom Selleck during the Rosie fiasco. He was there SCHEDULED to talk about an up and coming project. I am sure that he did not ASK to be there and get blind sided.