RSE, you might want to read the entire section we're discussing (or the whole ruling), when you have the time.
The Takings Clause is mentioned because Congress was in fact concerned about it -- hence §514, the part under review in the case. To resolve this issue, the URAA stipulated that in order for the copyright to be restored, the foreign copyright holder had to file a notice to enforce their rights, either with the US Copyright office or the "reliance parties" (people who were distributing the material in question). The reliance parties had a one-year grace period to distribute existing copies. Further, anyone who made a derivative work while the content was in PD has to pay a royalty for continued use of the derivative works.
From what I can tell, the opinion views this as sufficient redress for the reliance parties and that "the judgment §514 expresses lies well within the ken of the political branches." The dissent in this particular case does not suggest that the Takings Clause is a valid reason to overturn this particular law, and it doesn't look like the petitioners invoked the Takings Clause either. They were pushing a First Amendment claim in regards to their claims of "vested rights."
Meanwhile: Section III B, which we are both quoting, pretty much shoots down the entire idea that a member of the public can have a "vested right" in a public domain work. Here's the summary:
Petitioners claim that First Amendment interests of a higher order are at stake because they—unlike their Eldred counterparts— enjoyed “vested rights” in works that had already entered the public domain. Their contentions depend on an argument already considered and rejected, namely, that the Constitution renders the public domain largely untouchable by Congress. Nothing in the historical record, subsequent congressional practice, or this Court’s jurisprudence warrants exceptional First Amendment solicitude for copyrighted works that were once in the public domain. Congress has several times adjusted copyright law to protect new categories of works as well as works previously in the public domain. Section 514, moreover, does not impose a blanket prohibition on public access. The question is whether would-be users of certain foreign works must pay for their desired use of the author’s expression, or else limit their exploitation to “fair use” of those works. By fully implementing Berne, Congress ensured that these works, like domestic and most other foreign works, would be governed by the same legal regime. Section 514 simply placed foreign works in the position they would have occupied if the current copyright regime had been in effect when those works were created and first published.
Again: Public domain is
not a transfer ownership of the rights from the copyright holder to the public at large; it is the
cessation of any and all rights, ownership and protection. Anybody can do whatever they want, without limit, to a work in the public domain precisely because no one owns it.
Nothing in the Constitution stipulates that works in the public domain must receive certain types of protection, or that entering public domain is a one-way process that is inviolate and cannot be undone. As such, it is up to Congress to decide how to handle public domain works -- and, if required, re-establish copyright, as long as it doesn't result in perpetual copyrights.
You may not
like the idea that copyright can be re-established, but apparently in the US it's 100% Constitutional. If you want to change that, your only options are to convince Congress that public domain should be a permanent and irrevocable status, or to amend the Constitution.