I was wondering about that because as long as there is a category to query category permissions on you will have category permissions applied.
There is also a global permission that enables you to leave your categories open to view by guests without actually seeing the full ads or comments. That being the setting in admin => registration settings
Require users to be registered to view images?
If you set that to yes then guests trying to view the actual ad or comment are given the message they need to register.
This is my preferred setup because it allows guests to see there is content on the site to view etc but simply gives them a message saying they need to register. Marking guests as not viewing categories hides everything which of course can be a hinderance to your site. You want to tease people not drive them off. Just my 2 cents.