Amazon has a lot more clout with book publishers than Sony. Ebooks are the only books Sony sells, whereas Amazon is a major outlet for all books.
I think the lower prices at Amazon will end up forcing the rest of the market down, though. Particularly if ebook sales do well at Amazon. We've often argued here that if prices were lower, people would buy more ebooks. Amazon might finally put that to the test (though I think $6.99 is still too high).