Who says this has to be a battle used helmet? Stormtroopers are soldiers and soldiers are expected to maintain a dress uniform. Why couldn't that just be shiny plastic?
If you notice, RotJ stormtroopers had a duller look to their armor than those on the Death Star, as that wasn't a battlefield and seeing as it was the main project for the Empire, I would bet that things were supposed to be held to a higher standard.