> when one calls fsync, he expects [...]
> Fixing this deficiency through Python's exposed fsync [...]

I think so, too.
http://pubs.opengroup.org/onlinepubs/009695399/functions/fsync.html
even permits "null implementation"s etc. etc. etc.
(I like the last paragraph of "Rationale" the most.)
os.rst states for fsync():

    [...] to ensure that all internal buffers associated with *f*
    are written to disk.

If a platform offers the opportunity to actually implement the
desired behaviour then i would do so, regardless of what needs to
be done internally to achieve it.

(And the single question on apple is simply what to do otherwise
with that VMS/VFS bug for at least large sparse files.
I can only imagine adding multiple notes in the documentation,
here and there.)