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The collection is already being done by a private company, just not broadcasting it freely, you have to pay for access. And they do sell it on to other actors ofcourse :-)

Crowdsourcing data.

https://www.drnrecovery.com/

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.

In the US, that would probably be legal as long as you have the right to set up a camera at those locations (meaning, your property plus anyone else you get permission from).

At least Here in Germany it would be illegal. Even a dummy camera pointing at public area isn't allowed for a private person