I claim that anyone who buys books on the Kindle is in fact renting them, just for an unspecified duration that Amazon gets to choose after the fact. Amazon has already demonstrated that they are capable of remotely deleting books on people's devices, so your rental will last as long as it's in Amazon's financial interest for it to. That will probably be as long as Amazon exists as a company, but who knows.
The copyright cartels will never allow an extension of the customary "dead tree" unlimited right-to-lend to e-books, as that way lies the ultimate collapse of artificial scarcity, something they live in mortal dread of. I discuss this issue at length in my debunking of the B&N Nook's lending feature: http://www.loper-os.org/?p=54
