That's a spammer's definition. Everyone else's definition is that spam is unsolicited e-mail. Which covers most marketing e-mail, and not just the cold messages, but especially marketing e-mail from vendors you had interacted with in some way in the past.

> It worries me a lot that people clicking "mark as spam" on messages from legit companies because they subscribed to the newsletter will mean that my messages with important information (order confirmations, e-tickets etc.) will get blocked.

They probably didn't subscribe to the newsletter, they were subscribed, or tricked into subscribing. Either way, it's spam, and legitimate companies do not mix transactional e-mail ("order confirmations, e-tickets, etc.") with marketing e-mail.

FWIW, I'm one of such people clicking "mark as spam" on marketing e-mail, and I do it intentionally.