There's an interesting article
here on how publisher insistence on DRM resulted in Amazon's market dominance; mirroring the previous experience of the record industry and Apple. So not only has DRM been ineffective at stopping piracy, but it's actually wreaked real harm on the publishers by giving Amazon real power over them. What's that old saying about those who ignore history?
I'll also echo Xenophon's point, that the treatment of ebooks in U.S. law is at best unsettled; screaming that it's a license is useful if you're a publisher, but may not actually be correct. Amazon (and their lawyers) know exactly what they're doing when they allow the purchase of an ebook via a "buy now" button rather than a "license now" button.