Because, if you've ever created a forum, fast reply is an admin choice. When you open the page to create a new forum on a board, fast reply is not a delfault. That means he has to go through all the forums and make all the set defaults to fast reply.
If that makes any sense, there.
If you want to see for yourself go and get a free board from Invision and poke around. It gives you alot of understanding, and its fun..