Because it can’t be both ways. If tomorrow Facebook decides to only offer its app only through Epic App Store, consumers now have reckon with a privacy compromised app. And Apple now has to spend tons more resources playing a failing game of whack a mole closing all sorts of security and privacy holes that could have been avoided if the misbehaving apps weren’t allowed in the first place. Lots of people buy an iPhone for the walled garden, peace of mind experience.