|Other authors may do this, too.|
Granted, Rowling is not the typical author; far from it. Yet other big names (paging Stephen King) are probably looking at similar arrangements, or at least better terms. The big authors are what generate the most profits for traditional publishers, so this should be a wake-up call. Of course, you'd have thought the events of the last few years would have been a wake-up call, too, but you would have been wrong. Legacy publishers seem to be pulling the sheets over their heads and hoping the scary ebook monster goes away.
This is not to say that publishing your own ebook is necessarily the way to go. But every author now has a lot more options to consider, and some thought as to which path to take. There are many more options for getting your book to market, and that's a good thing. It's more complicated, though, and not necessarily easier. Heck, signing a deal with a publisher and then sending in your manuscript and waiting for checks is about as easy as it gets. Publishing it yourself is much more work... but you may make a lot more money, too, if you are successful at it. I advise all authors to study the market carefully; read Joe Konrath's blog, Mike Stackpole's website, and others.
Be prepared to do a lot of homework. Magic spells optional.