It's trivial to add a capacitor or hardware timer to illuminate indication light for some time after the camera loses power.
Wonder if malware could do reconfigure some chips (not necessarily macbook one) to go into parasitic power mode or something, that is when the power is supposedly off they keep running with power taken from some data connection.
Consider this sensor https://martybugs.net/electronics/tempsensor/hardware.cgi
If someone used such a sensor and overlooked the parasitic mode, then that could be used even when the power was physically shut off.
Even in the webcam case, powering the camera CCD would take significantly more power than is supplied by the data line. You can derive this for yourself without taking one apart by realizing that the power is supplied separately for a reason.
Further, it's not the case that this would work when the power is "physically shut off". If there's no power then there's no power.
But then in 2013 some researchers figured out that actually the camera unit is an entire system-on-a-chip, with a configuration register accessible on an i2c bus, so they could write some malware which first re-configures the camera to ignore the standby signal, and then turn it on...
The paper notes that many camera units have a separate power connection for the CMOS sensor itself, which would be more secure. And I hope later-model macbooks have fixed it. But I guess this shows that it possible to get even seemingly bullet-proof solutions wrong.
https://jscholarship.library.jhu.edu/handle/1774.2/36569
(As a more practical problem, I have also seen suggestions that it's possible to turn on the camera, take a photo, and turn it back off again too quick for the LED to be noticable, and if you do that several times per second you could capture low-frame-rate video without the green light, so even a hardware solution might not be perfectly secure.)