This is well documented in Apple's developer documentation and can be overridden with a custom loading image:
> When the system launches an app, it temporarily displays a static launch image on the screen. Your app provides this image, with the image contents usually containing a prerendered version of your app’s default user interface.
- App Launch (Default) Images: http://bit.ly/LMLbu0
It's been this way for close to 5 years now.