I think thats a really wrong definition of spam. Spam is untargeted junk from people you don't know, who are probably hiding there real identity using fake email headers etc. If it's a legit company with legit unsubscribe options, it's not spam.

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.

Spam is defined as unsolicited bulk email. Marketing is only spam when it isn't previous customers, or people who have specifically opted in.