I've never heard that described as the purpose of copyright law. Why would it be? In the absence of copyright law, all works would effectively be in the public domain.

It's funny, when they passed the Copyright Term Extension Act the opponents of the bill literally argued to congress that it was killing the American philosophy that the purpose of copyright is to encourage the creation of works that eventually fall into the public domain.

In the United States, Copyright and Patent law derive from the "Copyright Clause" of the Constitution:

    To promote the Progress of Science and useful Arts, 
    by securing for limited Times to Authors and Inventors
    the exclusive Right to their respective Writings and
    Discoveries.

This enables patent and copyright law. Patents do demand disclosure of details, but if you weren't worried someone would reverse engineer your product and get it to market before your patent expired, you'd just keep the details a trade secret. So I don't see how the disclosure aspect factors into duration.

If it wasn't already, the CTEA turned the length of copyright protection into a function of the American movie and music industries ability to lobby congress with sob stories. I'm not even trying to be snide towards congress with that remark, ask anyone in IP law what they think will happen next time Steamboat Willy is set to drop into the public domain.