I do not get that to work at all.
In all my tests with unsubscribed users, these subscriptions failed with the Admin message: "Subscriber blocked since unsubscribed. Contact the site administrator." People do not even see this message, so most likely they just leave it at that. I have double opt-in enabled and double subscriptions allowed.
The setting does however work for users who are subscribed at the moment.
But this is not how it's supposed to work, is it?