Trouble is, you are bound by the license agreement, not by the text on the buttons, as the Conditions of Use and Sale makes clear, which is linked to on every page of Amazon's site.
A cursory glance at the Wikipedia article on DRM suggests the law is favourable torwards DRM and all that that entails.
It's not so much about DRM per se but rather about what rights you can expect when "buying" an e-book.