Is the rough texture to reduce glint?
Yep. Any time you see a "shiny" helmet, it is actually the helmet liner, which were originally made from compressed, and impregnated paper fibers, and later from plastic or fiberglass. The liner by itself, offers a level of protection analogous to say a civilian hard hat. In garrison situations, MPs, drill teams, color guards, DI's etc. would wear polished, painted or even chromed liners. These were sometimes referred to as "parade helmets." The steel shell could not be worn by itself as the liner had the suspension, headband, chin straps, etc...
For example, you can see the difference comparing the following photos...