Baez is just doing his job. He knows they have forensic evidence, but doesn't know how specific it is or how potentially damaging it might be in any eventual trial. By floating an outrageous request like this, sure to rouse huge public outcry, he hopes to force LE's hand. He wants them to come out and be completely unambiguous about their stand on whether or not Caylee was in the car and is in fact dead. Now, don't pummel me. I believe she is dead and that LE has that proof. I'm just saying that I think Baez is trying to make LE release info to him prior to when they would be required to do so by discovery.
They don't have to do it, of course. He is gambling that they will not be able to withstand the goading and come out swinging in a fit of anger, having been finally frustrated beyond human endurance. I know it's working with me! I don't think it will work with them, though.