Sure, the intent was Amazon IT would eventually move to that system. But, they could let other companies work out the issues first vs. risking their main website. Just consider the risk for them if AWS goes down and take their own site outline while they are trying to sell high up time. That's simply a risk they had no reason to take.
PS: Remember, well executed traditional large scale infrastructure has higher up-time than AWS because AWS adds complexity.
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.
- the external customers get priority because they're real customers that pay real money.
- the internal customers get priority either because they're the "real" business of the company or because they have the right connections
Amazon uses a very tiny fraction of AWS, but I'm willing to bet that their bug reports and feature requests are accorded a fairly high priority.
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.