I have seen many threads in the Dominant Dungeon and the Q&A for Dominants forums that have slaves/subs post and reply. I feel that this site is somewhat of a support group where we can all get advice from other members of the society regardless of title. I love that there is a nice balance of Dominant and submissive replies. It really helps see all kinds of views on the subject.
As far as being punished goes for posting on a Dom/mes thread, my Master supports me being a part of this on-line community. We are not into on-line play. We both feel the real life relationship is far more fulfilling. So, He supports my involvement in the threads.
I understand what you are saying though. if a rule for your sub was to not post in a Dominants thread, that sub should receive a punishment for disobeying if they did not follow orders.