The end of apps as we know them imagines a future where a Notification Centre-like stream of cards was the main way of interacting with the apps on a device. It made a pretty compelling case, I thought.
I bought PCalc. The guy deserves a break.
Update 31 October: The decision's been reversed, calculators can now stay in the Notification Centre. A certain ambiguity in the rules is fine, necessary even, and the walled garden I still believe is also necessary; but it's all a balancing act. At some point opaqueness in the process becomes capriciousness. To err is human though of course, and mostly Apple seems to get this balancing act right, as they seem to have done here in the end.