> There is absolutely no need to put an unsubscribe link into a transactional email.
Agreed. rsync alluded to it below as well.
'unsubscribe'... from what? If I just bought something from service ABC, and I get an email from ABC saying "you just bought foo from us"... what would an 'unsubscribe' even mean? "Do not ever email me about this purchase again?" "Do not ever email me about future purchases?"