There is another, older, US based study that showed that many DOT only helmets were actually safer in normal street use than the Snell rated helmets because the type of impacts you expect on the street (head falling to hit street from the bike) are different that the type of testing Snell was doing then for racing applications, where they are attempting to protect from hitting things other than the ground at speed. But you would pay more for the Snell rating, and those helemts all charged a lot more money than just the Snell rating added.
Why do these "big name" helmets command such higher prices? It certainly goes without saying that it does not cost them that much more to manufacture them. And the UK study indicates that it has nothing to do with increased safety. So why do they get so much more money for helmets of comparable comfort and convenience features, and possibly even worse safety ratings? Because they can. Because of the premise in the line re-quoted from above: