Unsolicited commercial messages.
If the actual receiver didn’t solicit the message, it is spam. It doesn’t matter if the business is legitimate or not, if anything it makes it worse because the FTC occasionally does fine companies for sending spam.
Legally, you have to verify the email address before sending any further messages. If you don’t, you open yourself up to some serious fines if and when the FTC or whomever decides they want to make an example of you.
What about selling fake products?
Those persons give my address all over the place. Don't you understand the implication?
Also using unverified addresses by Paypal and other assholes is irresponsible and honestly I can't see why it isn't spam.
Edit: Add Netflix to the assholes list. I've just reset the password for another idiot.
The problem is the word spam. What you describe, misdirected mail from completely legal businesses is not really spam, even if it is junk for you personally.
There is aggressive marketing of very vaguely related products you have actually registered for. There is marketing of illegal products mostly using harvested addresses. There is phishing often using stolen addresses.
If you want to understand the problem as the OP obviously does just speaking of spam is not helpful.