Gmail ads have nothing to do with spam.
From a purely ad perspective, Google doesn't really care if the email is spam or not: they still display ads on the side.
From a purely ad perspective, Google doesn't really care if the email is spam or not: they still display ads on the side.
Now, such newsletters are very viable: often you subscribe to them when purchasing a product or getting a freebie. For many users (i.e. my parents) it is very hard to find out how to unsubscribe from them.
Currently, such newsletters are very effective. Sending them doesn't cost you much (from MailChimp to DIY) and you retain a lot of users, since they find it hard to unsubscribe (or don't bother). It doesn't really matter that Google adds advertisements besides such e-mail, because if I had a list, I have already slipped my advertisement into a user's mailbox for free.
Making it easy to unsubscribe from such mailings makes them less effective: it will become harder to retain a communication channel to potential customers, since more of them will unsubscribe. So, Google Ads become more attractive as an advertisement tool, evenmore because users cannot unsubscribe them (short of using an ad blocker).
What's worse is some places will re-add you to their lists every time you buy from them again. (I'm looking at you Moviefone.com!) I already unsubscribed. Why the @$#% are you re-subscribing me? It's not effective at all. It makes me never want to use your service again, which is what I've done with Moviefone.