Unless things have changed I'm not sure it's accurate that you only get charged for external traffic on AWS: I recall having pretty substantial charges internal to AWS just for traffic between different AZs in the same region, for example.
I think the parent is talking specifically about S3 rather than all of AWS -- S3 is a regional service and you can access it from any AZ within that region without additional fees. Cross-AZ charges are more commonly an issue with things like load balancers and EC2 instances.
Cross Region is common use case which makes S3 access from another region or if you are migrating painfully expensive when Compute is in multiple regions .
Cross Region replication is typical setup for many DR use cases and S3 will charge you Inter-region transfer cost for replication as well.
