If a group has a role assigned that just allows a user to view (nothing more) a Category, it doesn't give any message to that user about not having the required permissions to view the forums in that Category. If the same View Only access (nothing more) is granted to a forum, the return message to the user would be:
"You do not have the required permissions to read topics within this forum."
A similar message should be displayed to users with strictly View Only access to Categories as well so they know it's a lack of access and not necessarily an indication of there being no forums.
"You do not have the required permissions to view forums within this category."