I just checked on my system. If you pin a topic it will show up at the top of the topic list for those who have read-access to the topic.
If you make it a banner, however, it displays for everyone, regardless of their permissions. It is not clear to me whether this is a feature (if you didn’t want everyone to read a topic, why did you pin it globally?) or a bug (why let people see something in a category they don’t have access to?).
Will pinning it not accomplish what you want?
A violation of permissions seems a little like a bug to me, though perhaps it’s a feature request, as it does not interfere with use and this seems to be an edge case.