The problem is that you listened, but you did not hear. High availability is extremely subjective. There is a world of difference. If it goes offline, customers can't order. Amazon loses money for every second it is offline.
Customers can get frustrated and go to a competitor that is simply a click away.
Some of these customers might never return. Put simply, Amazon can not afford to have its ordering system offline for even seconds every day. You are going to find clustered database servers sitting underneath the site. You will find multiple clusters underneath the site. The application running on the site more than likely writes to multiple clusters simultaneously and a set of backup clusters is probably kept in synch via replication.