Does the 5th gen iPod touch rock when you put it on a flat surface because of the camera lense?
If you take any flat object, and at one corner place anything protruding from the back, the item will rock when placed on a flat surface and you press the front at a certain point.
The ONLY way to stop the rocking would be to have the rear of the case bent, so you have taken out the unevenness, or you place a tiny lug sticking out at the opposite side so that it's the same thickness as, in this case the camera lens.
At the moment, this is exactly the same as having a chair with equal legs, and making one leg a little longer. It will rock.
You could bodge you way around it by selling a case that stops the rock, but really I think that would be laughable to HAVE to buy a case so that the next iPhone does not rock on the table.
They could make the rear of the shell on the other side stick out a bit, but we've not see any evidence of that.
There must be some other reason for these mock ups. Just testing out concepts perhaps.