Here's an excerpt from Sprigman's posting that explains the case:
The Golan case challenges the constitutionality of the Uruguay Round Agreements Act, by which, among other things, Congress removed thousands of books, films, songs, and other creative works from the public domain and “restored” them to copyright. The Golan plaintiffs, a group of conductors and film distributors who used these public domain works, challenged Congress’s depredation of the public domain. The primary ground for the challenge was that Congress, by removing works from the public domain, departed from the “traditional contours of copyright protection” in a way that limited free speech in violation of the First Amendment. Limited how? By making the use of the former public domain works subject to the approval of the owners of the “restored” copyrights. By, in short, imposing copyright burdens on free speech where none had existed before.
The Golan plaintiffs’ First Amendment theory was built on something the Supreme Court said in Eldred v. Ashcroft. . . . . Where Congress does not act in accordance with history, but instead alters copyright’s “traditional contours”, courts must conduct a more searching First Amendment review to ensure that whatever Congress has done to the copyright law—which is, of course, a regulation of speech—does not burden speech in ways that cannot be justified.
Since Sprigman's post, there has been further commentary in the blogosphere. Here's a selection of postings: