It's a good point.

We don't actually commit to running infrastructure in one specific AWS region. Customers can't request that the infra runs exactly in us-east-1, but they can request that it runs in "Eastern United States". The problem is that with scenarios that might require VPC peering or low latency connections, we can't just run the infrastructure in us-east-2 and commit to never having a problem. For the same reason, what happens if us-east-2 were to have an incident.

We have to assume that our customers need it in a relatively close region, and that at the same time need to plan for the contingency that region can be down.

Then there are the customer's users to think of as well. In some cases, those users might be globally dispersed, even if the customer infrastructure is only one major location. So while it would be nice to claim "well you were also down at that moment", in practices customer's users will notice, and realistically, we want to make sure we aren't impeding remediation on their side.

That is, even if a customer says "use us-east-1", and then us-east-1 is down, it can't look that way to the customer. This gets a lot more complicated, when the services that we are providing may be impacted differently. Consider us-east-1 dynamoDB down, but everything else was still working. Partial failure modes are much harder to deal with.