A mechanical sliding piece of material without any electrical components at all must be considered simpler (and thus "less engineered") than an LED, mustn't it?

I would be way easier to convince that a piece of plastic is harder to hack than a LED. For instance, there would be no embedded microcontroller to subvert.

Also, USB is a bus, you can't splice an LED into the power line of the USB going to a camera and conclude that when the LED is lit, the camera is recording. There's a lot of other traffic that could be happening beween the host and the device, that doesn't mean the camera is performing any recording. The reverse would of course be true, though (no light = no recording).