It's up to the customer to plan for the worst too
To the uninformed....
I've worked in data centres where we've had N+1 and more resilience on every aspect of the infrastructure. However a well placed lightening strike, chiller leak, roof collapse, car crash or other act of nature/technology can cause the unexpected to happen.
Ultimately it is down to the customer to have business continuity/contingency plans for these events, however unlikely they may be. As Eric Hammond rightly states on his blog:
"A well designed architecture built on top of EC2 keeps important information (databases, log files, etc) in easy to manage persistent and redundant data stores which can be snapshotted, duplicated, detached, and attached to new servers."
If your application is mission critical then you need to spread it around.