With the traffic that we have on 4chan, it is often neccessary to leave behind the mark stating that a user has been banned, if only to quell further discussion on the prohibited topic.

You'd be surprised, we don't leave all that many posts with the USER WAS BANNED message (deletion is the most common way to deal with rules violations, as you'd expect), and it is indeed keyed to Off by default, so it is usually not used. You just tend to notice it more, due to the format and placing of the message.