You don't have to actually offer reduced redundancy to have multiple price points. Amazon could have added an option for big corporations to pay more for the increased SLA, the existence of the improved service agreement itself being the thing you're paying for.

Actual availability numbers is often secondary to Big Corp customers. The benefit of the CYA/SLA agreement, is that when shit does hit the fan, as the manager you get to shrug and say "well, we bought the improved SLA, not my fault Amazon failed to deliver".