IF, and this a only an if, there is some sort of surge suppression network built into the generator, the connection to the "building ground bus, GEC, or ground rod" is to provide a reference for the surge suppression network.
Do you know, or can you tell if any of the output power wires are connected to the generator frame, i.e. neutral to frame bond.
If neither of these conditions are true, then the only two things I could guess, is the desire to prevent the voltage of the generator frame from rising significantly above "earth" and to minimize EMI/EMC emissions from the generator itself.
