This is not about spam. This is about newsletters that users subscribed to, e.g. because they were interested in a product.

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).

I guess it depends on your definition of "newsletters that users subscribed to." I have never voluntarily allowed a company to add me to their mailing lists for any reason. I always carefully check or uncheck the appropriate checkboxes so that I don't get added to mailing lists. Yet I've ended up on literally hundreds of mailing lists over the years. Luckily, for these types of cases, where I bought something from a legit company, they honor their unsubscribes. But really, why was I subscribed without my consent in the first place?

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.