ADS-B Exchange use information gathered by private individuals and there is no law that require to hide something.
They also use radar data in North America, I guess that's the FAA provided data. If they wanted to they could probably do without it since they have so many receivers enabling them pretty good MLAT, but they probably prefer to not provoke FAA.
They have mentioned that they don't want governments to encrypt ADS-B or regulate it in other ways, so by self regulating they hope to be left alone.
"Information about a small number of flights may be limited or blocked based on requests from owners or operators via third-party services, such as the FAA LADD. Some high profile aircraft, such as Air Force One are not displayed. Most other aircraft subject to restriction are shown as anonymized by aircraft type."