That page (viewing a banned user's profile) is a HTTP 200 response. I have confirmed this on my test forum.
Note that a "no permission" page returns HTTP 403. A "no permission" page will result if the viewing user doesn't have the View or View user profiles permissions. In the case of a search engine you are dealing with the Unregistered / Unconfirmed group's permissions. This is a restriction on the viewing user, not the profile being viewed.
It should actually be returning a 403 and it does in my test.
Google is simply telling you that it's generating a 403 in case you're not aware and it's a mistake. A page that it doesn't have permission to view is the correct use of a 403. In the case of banned users, this is correct and expected.