Teams try not to show anything they don't absolutely have to, whether they have secrets there or not. It's just how they roll.
Mercedes, however, do have something called a "magic paddle". So they do have an actual reason for keeping the back of the steering wheel away from prying eyes.
Here is a short video clip of a reporter attempting to get Mercedes GP's team principal Ross Brawn to disclose the details of the "Magic Paddle": http://www.youtube.com/watch?v=luWuJx2Ukik
Mercedes, however, do have something called a "magic paddle". So they do have an actual reason for keeping the back of the steering wheel away from prying eyes.