Why unsubscribable mails have unsubscribe links

Sometimes you may find unsubscribe links in mails from various senders that you should not be able to unsubscribe from. It could be account registration or password reset mails for instance.

The reason

The reason for this can vary, it could be a bug in the sender program.

But it could also be that the sender is using a mail sending service and this refuses their mail sending customers from removing these links in an attempt to protect the service from sending spam.