Update (23.32): Matthew Collins sends a helpful clarificatory email:
First, thanks for the mention - hopefully more folks will visit the site.And Stephen Carlson anticipates some of these comments in a useful posting on Hypotyposeis.
Second, my original intent in putting the rider on the papers, however, was not one of claiming copyright or any such thing (you'll notice I don't say they are copyrighted, only that one should ask permission). Rather it was to reinforce the idea of scholarly courtesy in recognizing the provisional nature (both in print and online) of the Seminar Papers as a publication. We have had this approach to the Seminar Papers since its inception. I have seen (and heard) quotations of papers printed in the Seminar Papers used to argue that scholar X holds a particular point of view - when in fact in the final version of the paper published elsewhere, scholar X either takes a very different view or doesn't address the perspective quoted. The Seminar Papers has always been a provisional publication designed to stimulate scholarly interchange at the meeting through the circulation of papers in advance of the meeting. The fact that these papers were bound in a nice volume and sold made many assume they were finished and copyrighted products. If you look at the print versions of the past, you will note the only copyright claimed is for the printed collected volume. Authors still retained all copyrights and publication rights.