I'm not sure whether these guys are simply reckless or, instead, and hopefully, it's because they can't afford additional helmets. I suspect it's the former, but what do I know?
A number of years ago a friend and I hired a motorbike in Goa and were fined by the police as neither of us were wearing a helmet. We were informed by the police that at least one person on a motorbike must be wearing a crash helmet!