The outcome was not entirely unexpected. The article mentions that this is what Zimbardo originally intended. But I think even more important in this story is the great story-like quality of the result. It's a great story, and it "makes you think", as they say. One professor in this text is quoted actually saying that the story is bigger, meaning presumably more important, than the science. I remember similar reactions when some of Gladwell's shenanigans was uncovered. It just doesn't matter if it's true, if it makes a nice story. In fact, it's probably even better if it's not true.