Earlier quoted context omitted.
I'm not clear on this - Is Siri or Google listening for anything other than Hey Siri or OK Google?
It can't selectively listen. It processes all audio input for those key words. Once it hears the keyword, it then sends a recording of what you say next to a server.
(This is at least true for the iPhone, Hey Siri is recognized locally before anything is sent off to the net)
Re reading your comment, I guess you mean: the sound processing goes through the local microphone no matter what, even if the audio is only forwarded on if they lead with certain keywords, right? In other words, just like how every other voice activated system has to work. The privacy implications are that the equipment could be compromised to actually put the recording in longer term storage, like a bug or something.