Okay, given that they are not listening for any insidious purpose, they should all just add a legally binding, unrevokable clause to their all of their terms of service indicating that they will never sell any audio data or data derived from the audio data while the microphone is muted. Absent that, it is entirely legal for them to do so at any time for any reason with no consequences, so I see no reason why we should take the word of a amoral entity that pinky swears they will not do so when a legally binding statement is so much cleaner and more straightforward.