I hear you—we’ve been back and forwards on this a few times and ultimately the decision comes down to safety. Discourse treats every category as its own independent thing, so a sub-category isn’t a permissions “child” of its parent; it’s really just nested for navigation and organisation. We did that because admins often want a sub-category to be deliberately different from the parent.
We settled on a safety net, making it impossible for a sub-category to be less restrictive than its parent to avoid a situation where someone is locked out of a category but is able to see private sub-categories inside it. This way you get full freedom to make child permissions stricter, but you can’t accidentally leak content through a more permissive one.