I noticed that the phpBB users cannot be mapped onto the Drupal role anonymous.

In my environment, I only want official registrations processed by an admin from Drupal to phpBB and let anyone else use the forum by registering themselves. But the users registered from phpBB should not have any rights in Drupal other than the anonymous user.

Now, evidently creating a rol which is specifically for phpBB registered users and then giving this role the proper rights will solve this. But I would like to know why I cannot choose the anon user role.