If Amazon is down, it is losing money.

Amazon's infrastructure was designed to prevent downtime at all costs. This was primarily through services deployed across data center clusters to ensure that even if a data center (or two!) went down, everything would continue to work.

AWS grew out of Amazon's "excess" infrastructure that was not used after holiday peaks had ended. AWS follows the same "no down time" design and most AWS products offer service agreements with strong estimates about how many "nines of availability" the service meets.