My understanding is that the mic switch is a direct, physical, hardware cutoff, actually cutting the electrical connection, and that it’s actually located quite close to the internal microphones. In principle, you could have a cutoff switch design that would act as a similar hardware cutoff for both, but the layout would be a mess, because the jack and internal microphones are quite far apart.
I actually think that, with the way the switch is implemented, there’s no simple way to determine in software whether the switch is on or off, unlike the camera switch, where the camera device disappears. So even a software-based cutoff tied to the switch would be challenging.
Edit: actually, it seems s though I’m at least partially wrong about this. There is signal for the microphone privacy switch state that goes to the webcam interface connector: this is pin 26 in the pinouts document, though nrp has a pinout list with notes here.
For the FW16, it’s possible that a different design of expansion card audio jack could include a separate cutoff switch on the card, which would be comparatively easy.