As I've very clearly mentioned before, we are talking about kids here, not old, lazy adults. Requiring helmets for kids tells parents that cycling is dangerous (even though a real look at the data will prove it isn't any more dangerous than activities which we don't require helmets for). If cycling seems dangerous, parents are therefore less likely to allow their kids to ride bikes (to school for example). The kids grow up to become adults that were never turned on to cycling in the first place and very unlikely to take it up in adulthood. That is how requiring helmets cuts down on bike usage.