Unfortunately the world is nowhere near that simple. Big companies (and small companies that have been around for a while) accumulate multiple lists - they merge, they fork, they get transferred from one system to another. If you use absence/presence, then a merge of (unsubscribed || subscribed) will always result in subscribed - probably not what you want.

Sorry if this little bit of sausage-making bothers you, but it's pretty much how every mailing list works everywhere.