This is normal for many sites. It essentially brings you a single sign on experience. So now if you offer other things, areas, products for those forum users, they won't need to create another account.
You could use a query and create your own list by filtering out people who have not had any posts (UserForumPosts = 0). This will get your list smaller but not 100% true.
Other thing you could do is create a custom registration that is specific to those forum users. Add a new custom user setting called IsForumUser and set it when they register.
Most of the things I'm thinking of will require some custom code to get those users segregated.