Amazon are especially zealous of making sure no-one else can steal data from them. Not because of privacy concerns, but because they want to use that data first.
If this is the case I'm not sure what the fb/google/az connection would be. Perhaps, they could be buying the resulting data.
It would sort of makes sense if you installed a voice-note taking app and it asked for access to the microphone. However, when you are not using that app - is it still using the microphone? Who knows? I do get somewhat irritated with Android's inability to report which apps are doing what at any particular moment.
Full transcription is really difficult to do at the edge on battery power. It's either bandwith heavy or CPU heavy. Both of which nail the battery. The only really practical way I can see it being done is with keyword tracking, and then recording snippets either side.
But that still takes a reasonable amount of power to do.