It's not just AM radio but broadcast television, too. Since the ignition wires aren't sheilded, the voltage and current spikes on them cause radiated interference distributed into the airwaves. Given a points type ignition, when the points open, the coils dump their stored energy into the wires and spark plugs. The voltage only rises until there is sufficient voltage for the spark to jump the gaps of the plugs. The inline resistors soften the inductive spike on the wires, delay and stretch the current surge. For the plugs, it increases the duration of the spark. For RFI, it lowers broadcast frequency. This is important because, generally speaking, receiver sensitivy is more acute as the intended reception frequency increases.
It's interesting to note that the coils have a theoretical peak voltage capability that is never(or very seldom) fully achieved, because the plugs fire when there is enough voltage to bridge the gap. And, this stops further voltage build up in the coil from the collapsing field.
Probably more than you wanted to know.
Cheers,
