The FAA has a program called LADD (Limiting Aircraft Data Displayed) and high-profile individuals, etc can sign up to it. The major players in the flight tracking business like FlightRadar24 or FlightAware follow that. But sites like ADS-B Exchange do not adhere to that, so you can see a lot more flights that are blocked on the others. Also anyone with a Raspberry Pi and a cheap antenna can build their own ADS-B receiver and get that unfiltered data.
