Amazon occupies the historical feat of being the first commercial internet book reseller. I'm not too sure if they are still the biggest but my point is this. Being internet only it would be a matter of pride to sell e-books and being the biggest at it. A loss leader like the Kindle is the perfect stepping stone to achieve this.
As a reminder of this marketing concept here is a definition.
http://en.wikipedia.org/wiki/Loss_leader
My previous comment was a joke.
