Such live email addresses are very valuable to spammers. So this feature is susceptible to abuse by spammers providing the unsubscribe functionality, but only prioritizing the attempted unsubscribers higher in their mailing lists.
So built into their unsubscribe attempt, there needs to be a way to very strictly penalize those who pretend to allow unsubscribes while ignoring them to harvest more email addresses. Hence, they call it "unsubscribe and mark as spam" because any other email messages you receive from that sender will be marked as spam with extreme prejudice.
Google (or any other large e-mail provider, for that matter) could significantly decrease the value of an unsubscribe request to a spammer by automatically sending unsubscribe attempts every time an invalid e-mail address is hit with an e-mail with such a link. If spammers can't assume that an unsubscribe response means the e-mail address is live (and further, if the likelihood of a Gmail unsubscribe being a dead address is very high), then the scam no longer has any value to them, and they'll just stop doing it.
...in which case, it's still a useful thing to do, because it helps legitimate companies prune their e-mail databases of garbage addresses.
I understand the goal but the implementation is a problem.
Off course, there are probably problems with this, too.
The button should be "unsubscribe and mark all future emails as spam."
I'm just brainstorming about the implementation concerns behind this feature.
What bugs me is that some legit mailing lists are prone to receiving spam once in a while.
You completely ignore what I wrote and repeat OP's point without making an argument.
You aren't just labeling it as spam for yourself, you are labeling it as spam for the world. When enough people complain the company will automatically get black listed from that email service Do you think a big company like Amazon shouldn't be able to deliver order confirmations, just because a bunch of people labeled it as spam when they tried to unsubscribe.
Granted that's probably not a big deal for Amazon, since they have connections, but what about all those small startups? Think of a disadvantage they'd get, compared to big boys with name recognition and connections.
It's like that certified email scam. Big companies jump on board, because they can pay the fee. If that ever catches on, you have an additional barrier to entry for the small fish.
I really like pg's definition of spam.
The supplied link isn't always a one-step thing. (I am very annoyed when I'm asked to fill out my email address on the unsubscribe form since they know it!)
The way it is set up right now, it looks like the point is mostly to reduce the number of emails received. If you report something as spam, you won't see it in your inbox anyway, so the unsubscribe part can only help Google reducing the overall number of emails.