I'm an MLIS student in archives, and here's the basic idea behind archival copyright. I apologize if I'm not more clear, but cold medication does not do a whole lot for coherence.

Because of the nature of the items contained in archives, copyright remains a huge deal. It is related to the manner in which the materials are passed into the archive's care. Because the creator of the material held the copyright, it is partially given to the archive when the creator signs a donor agreement. The archive is then charged with the care and keeping of those materials, and must do what they can to protect their donors, and protect their donors rights.

It is easier for archives, many of which do not have the funds to hire a lawyer to survey their collections in great detail, to simply issue the blanket copyright and not get in trouble because of their copyrighted materials.