There is self enrolment without any key http://docs.moodle.org/26/en/enrol/self/edit - so users could re-enrol after logging in without any password etc. The snag with forum subscribe is that you need an eMail address, for this you need an account. When a user logs in they are no longer a guest and hence are not enrolled on the course through guest access, so can re-enrol.
Back with M1.9 I accidentally enrolled all course creators on a course without them enrolling. It was a mistake, just need to thing what happened as that would be the solution with the authenticated user role.