The airspace of a place is a commons, what happens in the commons is everyone’s to know.
Transponders are in planes mostly for safety. Their automated dissemination is part of the safety mechanisms of that transport medium and putting up with them (when required) is part of the privilege of using that public good. Similar to requiring drivers licenses to drive.
For one thing, it's different because there is no law that cars need an active transponder while operating.
But cars do have a license plate anyone is free to look at while they drive by so in that sense it's the same.
Flights supplying Ukraine were routinely top viewed flights on that website (they were flying to Rzeszów in Poland, so there was no real risk of Russian shooting them down).
AWACS planes and tanker flying in holding patterns over Poland, Romania and Baltic Sea used to be top observed planes on flightradar24 but I should be now working not looking through flightradar24 planes over Poland ( so I will link https://www.bbc.com/news/av/world-europe-60612255 that has video of inside one of them ).
Obviously planes flying combat missions are not publishing data there. Presumably ones training in restricted airspace are not either for also obvious reasons.
ADSB Exchange even has a ‘military’ filter to focus on them.
This is certainly not true in Europe, and in the US there's generally zero restrictions on publicly sharing any kind of PII.
But what's definitely not legal anywhere in the EU is to record unencrypted radio transmissions, use it to construct a database full of PII, and distribute it like Flightradar and friends do.
E: can't reply below due to ratelimits
>Hence why I said "in the US"...
Hence why I said "in Europe"...