The scariest part of the whole piece answers this question: Why are back doors with secret keys a BAD idea?

"... he had been left out of a decision that hurt users' security, the sources said. Due to a programming flaw, he told them hackers could have accessed the stored emails...."

The CEO of Yahoo must have known that this kind of scanning and storage puts their users at risk. She choose to do it anyway as being the path of least resistance against a more powerful adversary (US govt.). Bad judgement compounded by zero spine... Verizon looks like the perfect fit.