A sliding cover to a camera does not adhere to KISS from a user's perspective. It's actually an annoying additional step to using the camera - not to mention another moving part on a laptop that may break and/or jam. Plus it's actually less useful - if the sliding cover is open one probably still wants an LED to indicate whether or not the camera is actually active.
The idea of tying the LED to the power supply to the camera also won't work, at least for modern macbooks, because that camera sensor is in fact always active. It's also used as the ambient light sensor to automatically dim the display in low-light conditions. Of course, they could have used an additional sensor for that, but that would have increased the cost and complexity of the camera hardware.
The real issue here is much more general, which is that it should manifestly not be possible without root privileges to modify the firmware in any of the microcontrollers in a laptop (of which I'm sure there are several).