> Spam is by definition unsolicited email (often commercial in nature) from someone you never have had any sort of (business) relationship with.
Can you provide a reference to that definition?
I have checked on merriam-webster, oxfordreference.com, oxfordlearnersdictionaries.com, and wikipedia. None of them include the "from someone you never have had any sort of (business) relationship with" clause. Or even anything which could be read that way.
> Yes, it is super annoying that they automatically sign up your address for marketing
Yes, super annoying. And also spam. If I didn't ask for it it is spam. If they trick you to "agree" to it (for example by having a checkbox where checking the box means you don't want to receive emails) without you realising, that is still spam.
> there is a way to unsubscribe
Sometimes. Doesn't make it any less spammy.