Because, apparently, Facebook is not only about stalking the users and manipulating them into buying more stuff they don't need, but also about manipulating them into believing into the "right" stories and voting for the "right" people.
All of which makes censorship - both algorithmic and human-based - a must.
Another take-out from the article (for Facebook users): your perception of reality is being shaped by the kind of people who can't (or can't be bothered to) find a better job paying $15/hr.