I think you’re missing the point. This is not necessarily about users unsubscribing. It’s about the legitimate owner of an email address being harassed because others provided their email at sign-up, and you didn’t verify that the email was legit, and that it belonged to the user.
Worst case, I (as a hypothetical ill-intentioned person) could sign up an account using a targeted person’s email. Then, play with email notification settings a bit. They’d be getting spammed by every piece of activity on your site.
Worse, I could sign up another account, with a Junk email, and then send targeted spam/bullying as PMs.
Triple fun if I use two targeted emails, and make each think that they are being targeted by the other.
Tracking “evil me” down would be hard, and your forum or organization might end up taking the fall for any legal complications.