“Twentieth Century Fox television would not comment on whether Nancy Cartwright, the voice of Bart on ‘The Simpsons,’ will face disciplinary action following her voice message urging Scientologists, in Bart’s voice, to attend an upcoming conference.”
This is a legitimate contractual matter; Fox pays the lady quite a lot (I presume) to provide Bart’s voice, and use of the voice elsewhere reduces its value.
Why Scientologists think that using Bart’s voice might help them is another matter entirely. But I guess Bart Simpson is taken a lot more seriously in the culture than L. Ron Hubbard.