Rad am Ring. Awesome event. I guess it would be really epic to go around the Nordschleife on a bike, but also incredibly exhausting due to all the inclines.
I bet a lot more of them are drivers than you think.
Biking days at, say, Laguna Seca are very popular and usually at least half of the bikers there are drivers, the rest race fans of varying nature. A lot of the time those bike days are the only way to "walk" a track unsupervised and as a result drivers practically line up to do them.