"All works under copyright protection that are published in the United States are subject to the mandatory deposit provision of the copyright law (17 USC section 407).
This law requires that two copies of the best edition of every copyrightable work published in the United States be sent to the Copyright Office within three months of publication. Works deposited under this law are for the use of the Library of Congress"
I don't know how/if it works in practice, but if the copyright is supposed "To promote the Progress of Science and useful Arts", I think that it is perfectly fine for the LOC to guarantee that when the copyright expires at least a copy is still around.
This doesn't mean that it should provide universal access though, especially while the copyright is still valid.