But the microphone is always on if you enable voice activation, e.g., "Okay Google", "Hey Siri", "Hey Cortana". That recognition is handled in software, so no "hacker proof" hardware indicator can distinguish between the use cases.

"Hey Siri" works on desktop devices: https://9to5mac.com/2016/07/15/how-to-enable-hands-free-hey-...

"Okay Google" was removed from desktop: http://www.theverge.com/2015/10/16/9553051/ok-google-removed...

Yes, and I have my own issues with unprompted voice activation for that exact reason.

I think that having the mic light always on when you enable voice activation, would go a long way to showing consumers the risks of it. It makes it visually obvious that you are being recorded.