Currently subscribing to the forum means subscribing to the entire forum. That means you get notified of posts in all discussions. This works well in a classroom setting most of the time however on large sites like moodle.org its quite painful for a user who is only interested in an individual discussion to suddenly find they're being notified about posts in any discussion.
A site setting would be one way to resolve this. ie only allow per forum subscription OR allow either per forum subscriptions or per discussion subscriptions.