I'd never subscribed to it so why am I asked whether I would like to "Unsubscribe". Is it that the spammers got hold of my email from somewhere and "Subscribed" me automatically?
Shouldn't "Report spam" implicitly imply that it's not a subscription?
The worst offenders also make it difficult to unsubscribe. Hence, the report spam option.
A checkbox hidden somewhere that's enabled by default doesn't mean that "I have subscribed" to the mailing list, both in the common sense and also is prohibited in many legal jurisdictions (not USA, AFAIK), leaving a default checkbox simply doesn't count as obtaining consent, it is a well known 'dark UI pattern' that even the consumer protection laws have understood and explicitly implemented.
Such messages are just another kind of spam, and the right thing to prevent this, naturally, is to block the sender as a spammer. Underneath there is somewhat proper a mailing list that supports unsubscription, so an unsubscribe message can and should also be sent, but it doesn't change the fact that all the subscribers were added without their informed consent - if you built a system with opt-in by default, then you yourself built a system where it is impossible to say that you actually want to subscribe, the only provided checkbox then represents a choice between the default (no informed consent) and an explicit refusal of consent.
Out of 900 recipients, 1 marked it as spam, 18 unsubscribed. I believe people are very tolerant and they could use "spam & blame" a lot more.
The right thing is to not register me to these spam mailing lists in the first place. I did not register, ergo they are spam, and should be blocked from spamming other people. "Report spam" is exactly the correct behavior I intended when I clicked on it.