"The excess capacity story is a myth. It was never a matter of selling excess capacity, actually within 2 months after launch AWS would have already burned through the excess Amazon.com capacity. Amazon Web Services was always considered a business by itself, with the expectation that it could even grow as big as the Amazon.com retail operation."
This isn't true.
http://www.quora.com/Amazon/How-and-why-did-Amazon-get-into-...
"The excess capacity story is a myth. It was never a matter of selling excess capacity, actually within 2 months after launch AWS would have already burned through the excess Amazon.com capacity. Amazon Web Services was always considered a business by itself, with the expectation that it could even grow as big as the Amazon.com retail operation."