I would also like to see some sort of standard - like email header with link, that would unsubscribe you. Outlook/thunderbird/etc could just show button (probably next to "mark as spam" :)) and you couldjust click and be done. I think google tried something like this, but I've never heard of anyone else.
Companies can, and do, take out lists of subscribers and pass them to agencies to run campaigns for them, with potentially long lead times. Still stupid, but that's usually the reason.
Regarding standard, the List-Unsubscribe: header is sort-of that. It's not used much because there's little reason to - most clients don't do anything with them.
- We have a complex series of integrations (marketing automation -> CRM system -> app -> transactional e-mail provider) (as one example) where propagation isn't real time and could take a few days (if say, each of these were propagated on a nightly batch).
- Something could go wrong with any of these.
- It's better to err on the side of caution and give a super-conservative estimate and always beat it by a huge margin, than give an accurate estimate and occasionally break it (particularly with something as sensitive as unsubscribes).
"Any opt-out mechanism you offer must be able to process opt-out requests for at least 30 days after you send your message. You must honor a recipient’s opt-out request within 10 business days."
which means the recipient gets 30 days (from receipt of each email) to opt out, the sender has 10 days to process the opt-out.