Some of the commentary about why Google is doing this seems to me a bit misguided. Someone from Slate on Monocle’s The Briefing (sorry, missed the name) characterised this move as Google trying to make Gmail more useful for users and therefore more sticky.
I think the actual reason Google is doing this is to reduce or even eliminate a channel marketers can use to connect with consumers without going through Google. Subscribing to e-mail updates is a direct connection between consumers and brands. Google would rather be the middleman in that transaction, selling AdWords to brands and collecting a toll on all the traffic.
What makes me fairly certain of this analysis is that Gmail’s unsubscribe feature relies on the sender including the
list-unsubscribe header as per RFC 2369, so it won’t help with spam or with dodgy marketing e-mails in general, only with entirely legitimate and technically correct marketing communications.
I’m not on Team "Everything Google does is evil!", but that doesn’t stop me from taking a clear-eyed look at what they do.