It doesn't.

Our app, Greypad, is a PDF reader that uses the front-facing camera to detect when other people are looking at your device and then blurs the contents of your screen upon detection.

All apps, however, must first explicitly request permission from the user in order to access the camera(s).

Greypad: https://itunes.apple.com/us/app/greypad/id1450783054

Maybe not what OP had in mind, but there's also the issue of taking hidden photos of others. If you're holding your phone in my direction, I don't see what's on the screen, but an LED on the back could let me know that your camera is on.

Seems defeatable, unless it captures and matches the display output with the camera input, requiring some decently high correlation.