It turns out it's very easy to set oneself up as a copyright owner of public domain book:
1. Take the public domain book from Project Gutenberg or Google Books,
2. Reformat it as PDF,
3. Mark it with a copyright date,
4. Register it as a new book with an ISBN,
5. Submit it to Amazon.com for sale,
6. Tell Google books and all other sites providing the public domain version for free to restrict the access to it, as you're the copyright owner.
Apparently it works, and is currently used to effectively remove thousands of XVIII-XIX century books from public domain.
Also, it's claimed to be legal according to current US copyright laws (IANAL).
Though even if it is illegal, it would be effective due to the cost of making sure each book belongs in the public domain before the "copyright owner"'s claim can be rejected. Just like piracy works because the cost of taking all real copyright violators to courts is provibitive.
And here I hoped I wouldn't have to mirror the entire public domain on my hard drive....
Sources:
http://mikecanex.wordpress.com/2010/...n-parasites-2/
http://www.theregister.co.uk/2009/06/26/copyfraud/