I.e. lets say I do indeed not use it for transactional mail, but send a newsletter. How do I reliably fulfill my obligation to remove a customer from my newsletter database if they request so, if your service adds an unsubscribe link that doesn't inform me about the user request?

If I'm not using it for newsletters, you're saying I need a second transactional email system to communicate with the customers that pressed your opt-out link but I still need to communicate with?