This is how RSS works on phpBB. Although that software doesn't have individual feeds for forums, it does have an overall feed. The overall feed contains all posts, not just the first post from each thread.
The RSS as it currently stands in 1.2 only displays the content of the first post in a thread. Effectively all this does it highlight any new topics that are started on a forum but gives no indication of how the conversations in these topics develop.
Adding this functionality would mean that members could use RSS to keep up-to-date with reading posts.