This is the case for every database, but they still tell me what indexing options I have available.

With an RDBMS I know that I can use hash key indexes for fast lookups on an exact key. I can use B-Tree indexes for range or partial key lookups.

For S3 I know that I can do exact lookups, or prefix scans, and that scans occur in ascending order, but cannot be done in descending order, etc.

I know nothing of what lookup schemes are available to me in your application, and what kind of Big O profile to expect from each. It has nothing to do with whether it's a grocery app or a movie app.