Helmets are not designed for nor intended to stop bullets. I thought this was common knowledge...
You see SOF folks not wearing them all the time, they'll help you if something falls on you, or a grenade goes off, or you take a bump to the noggin, but for the most part they just make you feel safe while providing little to not protection against the things that will really kill you. Even when they are wearing helmets, you'll notice special forces helmets offer even less protection - essentially covering only the top and back of the skull, and mostly serves the purpose of being something you can mount equipment on. Any combat vet will tell you a helmet doesn't offer serious protection, even now with kevlar helmets.
Clearly, some of those legionnaires feel the protection offered isn't worth the cost to hearing and visibility, or they just want to look cool in their beret because they're legionnaires and it's a hard one to earn. Either way, it'll have little operational impact.