There's an important fact not mentioned in the article: schools and students often have incentives to take more AP exams. For example, I was in a school district where AP exams were free for students. And the schools often get special recognition based on the number of AP exams taken. Indeed, the oft-cited Newsweek high school ranking simply ranks schools by the number of AP & IB exams taken per capita; it doesn't matter what you score! (Citation: http://www.newsweek.com/id/39380). Clearly, students and schools have almost nothing to lose, and a lot to gain by taking the exams.

Given this perverse set of incentives, the only thing that surprises me is that the number of AP exams taken isn't growing faster! (We're still at about 0.3 AP exams taken per HS junior/senior)