A license plate is akin to a person talking on the street — audible to a few others, yes, but not meant for everyone perhaps. An ADS-B transmission is akin to someone putting the same words in a radio ad. Recording the latter seems completely different to me, privacy wise.
Person talking on a street wants to be heard by peers from one meter. Plane flying in the sky should be "heard" by other planes from tens of kilometers. It is a technical necessity, not a voluntary privacy surrender.
