American helmets are like in two stages. The inner helmet is like laminated material, almost like hardened hardboard/cardboard but shaped into helmet shape. It has a canvas head support system so that your head isn't in direct contact with the actual helmet. It's quite comfortable and holds the helmet firmly in place if it's a good fit. Over the top of the 'light' helmet is the steel helmet. This is much heavier and holds the lighter helmet even more in place!
So... I think that's why they don't need to do it up!