This procedure loads the battery very heavily for the second engine start, but it does prevent excessive loading of the "first" engine's generator during the second engine start. I suggest it's probably done that way to prevent excessive generator brush wear.
The later models (E & S) are started with the generators set to "ON", to come online as soon as the start cycle is completed, with the caveat that generator output is monitored and allowed to reduce below 160 Amps before starting the second engine. Presumably, the later models' generators are more substantial.