A justification for a law requiring motorcyclists to wear helmets most likely be based upon?

Helmets help prevent brain damage in case of an accident.

Do you know what medical personnel call motorcycle riders without helmets?

Organ donors.

Added: The most quoted reason is the cost to society of rescuing, treating, caring for, and rehabilitating riders gravely injured in motorcycle accidents. Most of this cost falls upon "society" in general since most persons injured in this fashion do not have the insurance (or inadequate insurance) to cover their medical expenses.