Here is how this works...Flight tracking websites allow and very often supply individuals ADS-B receivers that are connected to their servers. Initially, only 1090ES transponders were being tracked. Now, they are beginning to track 978UAT transponders as well, but so far, not as many are in use. At my home airport, HBI, there is a 1090ES receiver at the FBO. It will track your position even while rolling out after a landing. None of this has anything to do with the ADS-B towers...it is all a network of receivers installed by private individuals linked together. Whenever I fly using 978UAT, my flight is tracked in broken segments that represent areas where the 978UAT receivers have coverage...not nearly as widespread as 1090ES.