Lenovo put a little physical switch—they call it "ThinkShutter"—that serves to physically obstruct the webcam lens to prevent recording. It's supposed to have only two positions: lens obstructed or not. But if the user accidentally slides it halfway, you can still record video with the lens unobstructed but somehow the webcam LED turns off. It's because the ThinkShutter actually moves 2 pieces of plastic: 1 to cover the lens, 1 to cover the LED. But the piece covering the LED blocks it first, before the other piece of plastic blocks the lens. I discovered this accidentally yesterday while toying with a X1 Carbon... I am reporting it to Lenovo.
They fail to develop a reliable webcam indicator, and patch that with some half-assed attempt at physical view obstruction. The whole approach is a demonstration of bad engineering and unreliability. And that's just the part that became public.
The windows are there just to make the humans inside more comfortable, similar to how many people would be more comfortable without a camera pointed at them.
Flashing firmware is a big hill to climb for bad guys in most peoples worlds.