Exactly. No one here, least of all Tufekci, is claiming that YouTube has deliberately engineered their recommendation system to favor extreme content. The argument is that it's an unintended consequence of how the YouTube recommender's objective function interacts with human psychology: (1) YouTube trains its recommenders to maximize time on site. (2) It's a sad fact about humans that sensationalistic or extreme content is highly engaging. (3) YouTube's recommendation system thus learns to serve up sensationalistic and extreme content.