Amazon has no competitive consumer product to dropbox, and he rightfully might want to separate his work and personal data. Can't really expect one to use S3 as a dropbox replacement.
Does Dropbox not use S3 as its backend? I would not be shocked to learn that it does
perhaps you should be a little shocked to hear if they are using s3(they are not). s3 is awesome but it's not supposed to be used absolutely everywhere.
Whether dropbox uses S3 or not is irrelevant. Dropbox and equivalents are much higher level products not replaceable with an object store.
He is the CTO. He is in charge of Amazon Drive. He has the authority to implement any missing feature for his use-case.
Yes and as the CTO it is better to understand what products make sense for Amazon customers and implement those, instead of what he wants to use personally.