I can't find back the discussion on that topic but the goal was for a future version to allow conditional installation of dependencies depending on the selected storage. The thing is I can't find the documentation related to this subject on PyPI anymore… but I know that it's possible. Maybe @Jannis Leidel knows the trick :-)
Oh, I see, the backward compatibility can be tricky here given that current setups probably overwrite the PYTHONPATH to include de S3 module and will somehow conflicts with the proposed modification. Documentation must be updated to reflect that major change. Maybe the right timing to change the conditional dependencies' thing I mentioned earlier.