Creating a sub-forum is not much different from posting a new topic. On the forum page, a new topic can be more noticeable than a new sub-forum. So you shouldn't worry about users doing bad things with new sub-forums, because they can do more damage simply with new topics. Also, if you see a sub-forum that you don't like, you can simply remove it from your forum. We just want to give users this freedom, so that some good users may help you grow your forum in directions that you like. I hope this make sense.