The default setting, because 95% of the world will never change it. If Safari and Chrome and Android followed suit, the ripple effect would change a lot of things on the web. I'm not sure what the final result would be ... greedy people get to innovate too.

As I understand things, Prevent Cross Site Tracking allows all requests, but blocks all third-party cookies. This allows cookies on requests it makes, but blocks requests that match a commonly-used blacklist.