That's all stems from the David Letterman vs. NBC intellectual copyright fight from the 90's. This way, if the cast and Gracie Films ever part ways, Gracie Films can recast a perfect vocal replacement and not be sued by the original actor. A few years back (1998 or 99), when the cast of The Simpsons held out for a bigger paycheck, Gracie Films held open voice auditions in L.A. and San Francisco for possible replacements. I was lucky enough to audition for one of the casting directors over the phone (impersonating several of the minor characters such as Apu, Chief Wiggum, Monty Burns and Waylon Smithers). I received a call back for an in-person studio audition, only to be told 2 days before the audition that the cast, FoxTV and Gracie Films had come to a settlement...

I know the cast and Gracie Films went through the same thing a few months back, and that the cast again won a majority of their demands. Of course, this was before the CBS decision to come down hard on the holdouts for CSI. I think now that CBS has shown it won't tolerate renegotiations of existing contracts (at least for characters they deem non-threatening to a series if they are replaced), they have opened the doors for other production companies to do the same. This should be much easier when it comes to vocal talent. There have been stories recently that Harry Shearer is not happy with his roles on The Simpson's. I'm not sure if this is another holdout tactic, but I wouldn't be surprised if his contract was paid off and a replacement(s) found.