Constant change is a hallmark of a totalitarian régime. Chairman Mao’s permanent revolution. Or my old sociopath boss, who used to just make everybody move around the office every few months just because he hated the idea that anyone was feeling comfortable or enjoying their work.
Facebook keep changing privacy in the hope that you’ll give up trying to keep on top of it, and just let everything go to the default. When you do that, they can sell more of your data. And keep changing the privacy setting so that more of your data is available by default.