That's for direct impact. A lot of motorcycle accidents involve glancing blows where a rider's head bounces or slides along the pavement and for those situations a helmet can save your life even at highway speeds. I personally know a rider who was sideswiped at 80km/h; his helmet was destroyed and his collarbone was broken but he did not sustain any kind of head injury.
That being said, I wear a helmet but wouldn't make it mandatory. I would make eye protection mandatory, though, because a rider who's blinded by a bug is a menace to everyone around him.
FMVSS 218 have VERY specific standards.
BTW the ONLY legal standard for DOT certification is FMVSS 218. Snell is legally irrelevant.
the 14mp PER FMVSS 218 is the impact test.
FMVSS 218 has no weight or neck standards. There are significant variables that make any one helmet style insufficient for all uses.
However that is a CHOICE.