It's publicly available data. His plane's location can be found on a lot of websites if you know his tail number, which is also publicly known.
Look up ADS-B. All planes over a certain size are broadcasting their information all the time.
All this guy did was automate collecting the data and estimating fuel usage.
I suppose the thinking is that the skies belong to all of us and are publicly regulated, so we get to know how they are being used. People who don't like it don't have to own a private jet I guess. They all knew the rules going in.