The comment I wrote in response was 'may simply mean missing the tiny checked-by-default "don't not subscribe me" checkbox'. That doesn't describe
all cases, just many common cases, hence the "may". If the deal is "free music if you subscribe" or "sign up to our newsletter and get free music", and that's presented in an obvious and non-deceptive way, and people sign up anyway, then sure, that's a legitimate subscription. (And people will
still mark it as spam if they don't actually want it, and that's something to take into account when thinking of designing a system like that.)
And if the deal is "free music! Also, check this clearly identified box that's currently not checked if you want to subscribe to our newsletter", that's also a legitimate subscription; some people will still mark that as spam, and that's just something newsletters have to deal with, but I have marginally more sympathy for that case because spammers have somewhat ruined the concept of expecting reasonable unsubscribe links in unexpected mail.
But if the deal is "free music! (well-hidden fine print: leave this box checked to subscribe to our newsletter)", and someone misses unchecking the box, that's spam, and it should get marked as spam, and that newsletter should have serious deliverability problems; that's spam filtering working exactly as it should work.