Saying "unintentionally" here is like saying you unintentionally stole someone's TV when they gave you their key to walk their dog. It takes extra work to upload those contacts, which means several managers and developers decided to do it and then spent time implementing it. For the FB employees reading this: what is your tipping point? Would you say no to that assignment?
It also takes extra work to ask consent. You build it. You don't notice that your confirmation screen fails to trigger. You've just unintentionally uploaded a bunch of data without consent, when your intention was to do it with consent.
It's still pretty darn negligent, but it's easy to see how it could be done unintentionally.