I think the problem is that while engagement may be the source of profits that is short-term thinking. If joy based engagement drives profits and keeps users on your site long-term it may be worth it to take the short-term revenue boost that anger-based engagement brings you if it slowly drives people away from your site.
Of course it isn't completely clear if anger-based engagement actually drives most people away. In that case it is actually Facebook's fiduciary duty to capitalize on it which seems to be a failing on our society.
Facebook is still not shrinking in the US in terms of users. So even if there is a long term cost it seems well beyond the time horizon companies care about.
Of course it isn't completely clear if anger-based engagement actually drives most people away. In that case it is actually Facebook's fiduciary duty to capitalize on it which seems to be a failing on our society.