As I understand it, subsections are really just "special" tags that are for whatever reason shown in the forum root. However, this is not immediately obvious especially for us old folk who are used to real subsections.
As a user, if I click on a subsection link and then decide to post a thread, I'd expect that my thread will be added under that subsection. At the moment this won't happen unless I include the right text in the thread title or explicitly add the subsection tag to my thread. I think that if the user has navigated to a subsection and opened the new thread form from there it should include that tag automatically in the list of tags near the bottom of the form (therefore still giving the option to remove it). Otherwise it will appear to get lost in the aether.
More about :
usability posting subsection add tag automatically