Your distinction well-noted, a file server still makes a great name for the service, considering its end purpose is serving files, and the fact that it's a "service" implies an interface of sorts apart from a standard file system.
It would except for the fact that "File Server" has been a commonly used term for a long time that hadn't ever been previously used to describe the technology that S3 actually is: an eventually-consistent distributed object store with a simple network API.
It's not like "Simple Storage Service" is weirdly obscure or anything.
