According to source code, Posting message and posting thread rely on the the same right Forum:POST ?!
Moreover it seems obvious that if you are allowed to post a thread, you should be allowed to post message in this thread… The opposite sentence might not be true (allowed to post message implies allowed to post thread)
Therefore, i think it was really a bug (post right was not inherited) which was fixed by Florent today. But another behaviour could be designed by adding a new right.