How did Apple do that? I've seen the documentation while building stuff with Xcode but I never saw anything that would have actually forced my app to behave (outside of iOS).
It sounds like you're not asking about technical enforcement — as opposed to prodding, which is what I was talking about. You're right, though, they do also apply pressure through technical means. They did this at design time, by thinking through what the default QoS will be when it is left unspecified.
