Apple has always been a company that stresses the details. Everything down to the tiniest pixel is highly scrutinized to perfection. That’s why it doesn’t come as a total surprise that Apple is going after developers for using the wrong type of iPhone mockup to promote third-party apps on the web.
Cupertino doesn’t like the idea of developers using white iPhones to show off their work in marketing materials. It’s a black iPhone or the highway.