Why Live view slaps the mirror back down when you take the shot as oppossed to leaving it up and actuating shutter curtain?

It is my understanding that in regular conditions when using an SLR camera (No live-view) , the camera lifts the mirror to let the light hit the sensor. But while in Live-View, the mirror is already locked-up, so, at the time you press the shutter button, why the need to lower the mirror, and immediately lift it again just before exposure?