I think they're arguing that children should be made to wear helmets because won't somebody think of the children.
The same argument applies to children of course - where do you put your helmet after you've cycled to school? Attach it to the bike loosely and someone will steal it, it's too big to carry in your rucksack all day, and you don't want to have to go back and forth to your locker potentially on the other side of school if you have one at all.
Also if it's locked to the bike you would break the lock and steal the bike. The helmet is the cheaper part. Helmets are often designed so locks can go through them.
The same argument applies to children of course - where do you put your helmet after you've cycled to school? Attach it to the bike loosely and someone will steal it, it's too big to carry in your rucksack all day, and you don't want to have to go back and forth to your locker potentially on the other side of school if you have one at all.