This system is called ADS-B. It's a peer to peer communication between airplanes where each plane will share its own data (flight level, speed, coordinates, flight number, etc.) and data from other planes by broadcasting it on same frequency as others. Result of this is that all the planes know where other nearby planes are located and their heading thus providing additional safety in avoiding collisions and other issues.

Data being broadcast in all directions means us on the ground can listen to them as well, which is what some sites are doing. Add internet to that mixture and we have a world coverage of every plane on the planet. You might wonder why data is not encrypted or something similar. There's nothing that can be abused about the data and all this tracking helps enormously with resolving incidents and other useful functionalities like precise arrival estimates and similar.

Now, since every airplane has a registration number, people figured out which plane was hers. Just like they know which plane Musk uses and many others. Data is recorded and known and completely legal, helpful even. If she has issues with this fact solution is rather simple. Get a charter flight or rent a plane and no one is the wiser. This is just rich people entitlement and the only way she can "win" this court case is to go a step further in stepping over the line which is to drown the poor man with legal expenses.

Edit: Oh, also location of boats is known. Not all boats, but commercial ones for sure. Their system is called AIS.

Part of the answer is "it's always been that way". All this is done in coordination with the FTC and there are certain frequencies assigned for aerial radio communication. It's not just done on 2.4GHz to be cheap like Wi-Fi / microwaves / etc.

Since the long-long-ago planes have been required to have their registered tail number visible. So if you do something not okay someone can find you and talk to you about it. After all cars are (mostly) limited to roads, boats are limited to waterways (and also have to have a name/number visible somewhere), planes can go anywhere and be annoying.

Radio communication to/from planes is just 'in the clear'. It's not encrypted because there isn't/wasn't a reason for it to be and for a long time it wasn't even possible. Encrypting it offers no benefit and many potential downsides. It's one more thing that can go wrong when you have a problem and need to talk to someone.

When tracking was upgraded to transponders ( a small radio dedicated to sending out info about who you are ) all that just stayed the same. They transmit a code you set and your altitude (and other info including tail number for newer ones/requirements after 2020 (just a coincidental year, the upgrade was planned years in advance)) so Air Traffic Control can say "Bugsmasher 123 turn left please". Radar is expensive, a device in the plane constantly broadcasting "plane here" is significantly simpler and cheaper to track. Most airports don't have Primary Radar.

ATC doesn't care about your tail number for tracking anything other that what to call you to get your attention on the radio. For your current flight, if you are using Flight Following (talking to ATC so they know where you are) or filing an Instrument Flight plan, they give you a semi-random transponder code for that flight. That is what they use for logistics. They don't care who you are, they care where you're going and when you're going to get there.

Tracking a tail number isn't much different from the <1900s version, which would be "weird the Mayflower is in port in Boston" or something.

Safety against air collisions.