That's like complaining when someone redistributes something you gave them under the condition than they promise not to.
I can theoretically repost any "private" content shared by my Facebook friends. Is that a breach of the TOS? Is that illegal? Is that Facebook's fault?
So technically, one person of the group makes the post public by clicking on the Storify button? Not sure if it's a FB problem, people just need to be careful about what they post