When you enable Windows sharing in OS X, only the user’s public folder is normally shared. A while back someone posted a couple lines to type in Terminal that made all volumes shared. I have tried the search but to no avail.
This is a tip for Jaguar, but it might still work:
Only the user’s Public folder is shared to guests. If you browse as the admin user of the machine in question you can mount any of its volumes.
But yes, OS X is annoyingly limited as far as sharing some (other) specific folder or volume to users or groups of your choosing. OS X “thinks like a server” and expects you to set up user accounts right there on your own machine that correspond to who gets to have remote access to which volumes. And non-admin users are expected to confine themselves to their own personal folders (as defined by the local machine account for them) and guests to the public folder.
SharePoints (freeware) used to provide a way around this; haven’t been in an environment where it mattered since long enough that I can’t testify that SharePoints would still work nowadays.