​This may not be a valid assertion.  It depends on whether the publishers took the trouble to contact the author's literary heirs and negotiated a transfer of copyright.  I'd be surprised if that happened. 

When publishers print a (C) statement on a book or article, it isn't always correct.  It's happened to me at least twice that a publisher has asserted copyright over my work when they have no right to do so.  First, with my book Metarules of Paninian Grammar.  In that book, there are two copyright statements, mine and the publishers.​  I gave no rights to the publisher, Forsten, and their (C) statement is legally invalid. I was unpleasantly surprised when I saw it, but the book was then published and there was nothing I could reasonably do.  The second instance was my article "A Body of Knowledge" published by Brill in Asian Medicine.  Brill was paid €400 by UCL, my then employer, as a fee to make that article Open Access.  They printed the article and put it on their website with an illegal "(C) Koninklijke Brill NV, Leiden, 2008" statement: I had never transferred my copyright to them.  That (C) assertion of theirs is invalid.  The very same paper is also distributed in PubMedCentral, with the correct Creative Commons license.

You have to watch these publishers like a hawk.  There's real money involved, quite apart from ethical issues. 

Dominik