Michael Reinhardt in an interesting thread in TidBits discussion board TidBits Talk summarized the rationale for the challenge to the Sonny Bono act. Basically, as he points out (and I’m cribbing here), Congressional power to enact law is based on specific permissions or “grants” in the Constitution, primarily found in Art. I, section 8. The clause which authorizes copyrights is Art I, Sec. 8, Clause 8: “To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries.”
Reinhardt adds “The crux of the argument being used to challenge the Sonny Bono Act is that extending existing copyright terms does not promote the progress of Science and the useful Arts, and therefore Congress had no authority to pass the act (the act is unconstitutional).