I don't think you can make a blanket judgement on this issue, as it depends rather on the relationship between artist(s) and sponsor, and what arrangement they make between themselves.
Why should it remain independent of the sponsor? You pay for the event, you get what you like. No-one is limiting what the artist does with the rest of his time but if you give a comission you are paying for a product. If you then don't like it you may have no taste but you shouldnt have to fork out for it.