Earlier quoted context omitted.
This is a persistent myth that needs to die, AWS was not built to host their websites, and indeed did not do so for many years after it was established. AWS wasnt their "spare capacity" nor was it them exposing their internal systems to the world so they could sell them off, It was developed from day one as a public facing service.
AWS was built to host Amazon. Sure, it was a public facing service first , but AFAICT Amazon.com was always the primary target customer, and probably still is. AWS was built based on the needs and experience for hosting Amazon.com.
To make the statement today that Amazon "probably still is" the primary target customer betrays a lack of understanding of the scope of AWS and IaaS in general.
[1] It might be even 8+ years. I don't recall when it officially started so using 5+ as a very conservative estimate.