Actually, most accidents happen on the highway, but many accidents do take place in the home, and many places have a high incidence of pedestrian injury as well as injuries to cyclists.

The argument for helmet use in the home, or on the sidewalk might sound silly to you, but there's no real reason we shouldn't have laws for those situations if we feel we need laws to force cyclists to wear helmets.

To the extent these people suffer head injuries they'd benefit from wearing helmets - wouldn't they?

The point is that there's no real reason to discriminate against cyclists. Either helmets work, or they don't. If they work and they help with head injuries then you need to apply the law equally to all who are vulnerable to head injuries - not just the people who choose to bike.