Slate's article on the missing "last three volumes of the new edition" of Proust explains:
"Only the first four volumes of the new translation—from Swann's Way through Sodom and Gomorrah—are available here. For this we have Sonny Bono to blame. Just before he died in 1998, the congressman sponsored a bill to extend the term of copyright by 20 years: According to the Sonny Bono Copyright Act, passed later that year, rights would expire 95, rather than 75, years after an artist's death. Since Proust died in 1922, only those four volumes first published during his lifetime had passed into the American public domain by the time the Bono Act became law. It will therefore be at least 2018 before readers in the United States can find the final three installments of the new translation (The Prisoner and The Fugitive, and Time Regained) in their local bookstores."
The Literary Saloon examines the grey:
Now, we hate the Copyright Extension Act as much as the next guy (it's an outrage!), but the case isn't quite as black and white as he suggests. For some reason Matz does not address the fact that the American publishers could very easily get the books into bookstores: all they have to do is compensate the copyright holders (the heirs of Proust's literary estate). That's what they do with almost all the other books they publish -- why don't they give that a try here? (The benefit of waiting until the copyright has expired is that at that point they don't have to pay, meaning they get to keep all the money they earn, but that's the only difference.)
Paying copyright holders for books is the common practise -- after all, relatively little of what's published is out of copyright -- so why not here? Just because the British didn't have to the Americans won't either?
There may be difficulties we don't know of: the copyright holders may not approve of the new translations and thus might want to prevent them from appearing at all costs (i.e. would not agree to allow American publication at any price), or they may be making unreasonable demands. But it shouldn't be that hard to resolve these -- if the publishers were really interested in making the books accessible to readers. But since publishers generally don't really seem to care too much about serving readers, and their accountants can make a good case that they'll make a lot more money by waiting we're not holding our breaths.