You're right, but how to do it otherwise? With the camera, there is at least the option of installing a mechanical shutter that can be easily seen (and understood) by users. With audio, I can't think of any similar solutions at the top of my head.

Of course, technically, not even this hardwired circuit is complete proof that there is no trickery going on. If Amazon really wanted, they could simply hide a second mic at some other place inside the device.

I think the only solution that would really solve this was public auditing of the whole device.