Why? When a user blocks a whole instance generally but then specifically subscribes to a particular community on that instance, why would you expect the subscribed community to be empty?
Because you blocked the entire instance these communities are on. I feel that is the most obvious behavior. Subscriptions don't really factor in here as they aren't "whitelists" for blocks.
Indeed if you have a false advertising scenario [...] You seem simultaneously acknowledge that there is a deception
Advertisement? Deception?
It's a technical feature that doesn't behave like YOU were expecting. There is no false advertising or deception here ... calm down.