Well, for starters, all customers that are not in us-east. Such as US west coast, Europe or Asia.
Besides latency, data protection issues also become a problem when you cross borders. My employer's (SAP) cloud platform advertises as a unique selling point that they have data centers in a wide variety of locations, so that a customer's data never has to leave their jurisdiction (which is important e.g. for government and its contractors).
And that's before we get to high-availability setups. Remember how AWS us-east was down just a few months ago?
A business needs their S3 component highly available. If they don't have a DR plan in place, and a massive S3 outage (like the one that happened a couple months ago) occurs, they're fucked.
It is very, very rare from my professional ops/devops experience to see an org built to survive a region outage; the tools are there, the money/business case is not.
It doesn't have to be HA and replication. Sharding by customer would mean a region outage takes out only 1/N customers. That wouldn't have a higher cost, other than whatever dev effort is needed to work that way.
