As someone who operates two Nest cameras (one doorbell, one camera, both record audio) in California with clear line of sight towards public areas (sidewalk and street), I would really like to know what sources make you believe this is illegal. I find that claim very hard to believe.
If you are concerned about this being an issue for you. Place one of the window stickers that came with it in a front window. That should satisfy any need to notify others of possible surveillance.
