If you buy a paperback book you're purchasing the physical materials in the book (i.e. the paper and ink), and a license to the copy of the material in the book. You can't do whatever you want with the content - e.g. you can't copy it and sell it on.
It's only now that the materials and the content can be explictly and easily separated that we see this logic actually apply in more meaningful ways. This is going to apply to more and more things in the future (e.g. cars, phones, etc.) so we better get used to it, unfortunately.
But you can share with others. You can even sell the book.
It is very convenient to have hundreds or even thousands of books in a tiny device, especially for someone who has very little space to keep physical books (and those who move a lot). While nothing can replace physical books, ebook readers are a fairly good compromise. Plus, it is possible to reduce the cost of books (and less dead trees). But we can't share, unless we share the ebook reader device itself :(
As with most things, humans will put commerce first, and I don't see this situation changing anytime soon. Amazon has won the ebook reader race and they are likely going to stay there for a while.
The important difference is that the only "license terms" for paperback copies are laws designed to protect publishers, whereas digital licenses can be as arbitrary as a publisher chooses to as long as they don't conflict with laws designed to protect consumers.
It's only now that the materials and the content can be explictly and easily separated that we see this logic actually apply in more meaningful ways. This is going to apply to more and more things in the future (e.g. cars, phones, etc.) so we better get used to it, unfortunately.