I've been thinking about this, and IvanB and Blackcatter, I believe that you are both mistaken--for this reason--

I have had some experience in dealing with employment law, and, while it is true that you can terminate someone for no reason, it is also true that if there is a reason given, it can't be an illegal reason, such as race, creed, sex--I think that terminating someone for the excercising the freedom of expression , particularly an artist, who you have actually contracted with to express themselves, falls under the catagory of an illegal reason--

That is if this is an employment situation, but it isn't--it is a contract tor service, which would means that neither party can unilaterally terminate "at will" , like you can for employment--There are a lot of standard, customary reasons that are acceptable for terminating a performance contract, but it seems to me that you would have a hard time convincing a judge that it was reasonable to terminate a contract(that is, cancel the upcoming gigs) because someone objected to political/topical joke, particularly when you knew that the performer did political/topical material--

I am not a lawyer, but I have read a number of court decisions on freedom of expression issues, and the court is very, very, concerned about protecting these rights, and takes a very dim view of any actions, particulary arbitrary and capricious actions, that curtail them--