One of the biggest reasons you go for a cloud is because you don't want to deal with reliability & scaling issues, and there's a premium price attached to that. I think most companies using S3 in this case believed they put their eggs in different baskets when they put their data in there.
I can't believe anyone would have thought a dependency on a single AWS region, or even single service provider, would count as having eggs in different baskets, at least, I really hope nobody could think that!
I suspect though that most people affected deemed the risks and costs of failure low enough to be acceptable, and for many people it still is - even with this outage. But that's a conscious decision, rather than plain ignorance.
