That's a bit of a misconception. From what I understand, Amazon's non-AWS branches don't get deeply-discounted services from AWS. There is a discount, but it's not enough to turn dark skies into sunshine and rainbows.
Amazon tends to want every part of itself to be in ship-shape, and giving itself a massive discount would discourage efficiency in non-AWS parts of the business.
Disclosure: neither a current nor former Amazon employee.