You can go back to the previous screen (before the camera came on in Reverse gear) by pressing the Return button on MMI, or a simple press of the MMI dial cancels the camera.

I can understand your frustration in part, as you cannot adjust the radio station for example, while the camera is on. But as others stated, it helps when going from reverse/forward/reverse in parking situations to leave the camera image on screen.
I believe the manual gives a brief explanation to Audi's logic as well...

If you want to adjust the Radio Station when the camera is on simply use either the wheel controls or the next or previous station buttons.

Yeah, but unfortunately you can't set the radio to default to preset mode, so hitting next station will likely take you to a station you don't listen to unless you think to put it in preset mode before putting the car in reverse.

'Tis a minor issue to be sure, but annoying nonetheless.