Joel Johnson from NBC New on the licensing around digital purchases from Amazon:

The core issue might actually be a simple matter of semantics: when we click a digital button that is labelled "Buy," we expect that we're actually buying something. But we're not buying anything, we're licensing it. Just last year, the Supreme Court ruled that the first-sale doctrine does not apply to software -- or e-books. Or apps. Nor pretty much everything you "Buy" online that doesn't get shipped to your home in a cardboard box.
The perpetual tug-of-war between content wanting to be free and companies trying to lock it down continues.