Tracking jet flight locations of anyone is legal?

The thing is these tools don't track people; they track planes.

And only planes. You don't know who was in the plane, or where they were before or after the flight.

Yes. You just need a mapping of airplane designations to prospective users, because the information about plane position flying in certain "modes", let's call them, is public so long as you can receive it (it's necessity for making flying safe)

Definitely. https://theaviationist.com/2011/11/24/af1-adsb/