As I understand it, the charging circuit is wired through the "gen" light on the speedometer. If the bulb burns out, the circuit is broken & the charging system stops working. The resistor is there (in parrallel) so that the charging system keeps working just in case the bulb burns out. Pretty funky, eh? Check out Mike Dailey's site. He's got the schematics/electrical diagrams that explain it all.
Paul