Bravado? Nah. Plenty of people don't wear cycling helmets (although whole different can of worms) - some people just don't like wearing a lid. I've played with/against plenty young enough that they grew up having to wear one and still prefer batting without. What's an acceptable risk is different for different people, no point getting righteous about it when it's not your head that's affected.
Hmmm, I'm not getting righteous about it... I'll give you an example. A couple of seasons ago my brother who is a much better player than I am insisted on not wearing a helmet. He was 100% confident he could deal with the bowling our league had to offer. I didn't like watching him go out to bat without a helmet. It wasn't my head but it was my worry that sooner or later he was going to wear one. Every game and every net session I kept on saying to him "Where is your helmet, why do you think you don't need to wear one, even the best players can get hit why does that not apply to you?" I walked into our club one day and his eye was a mess, red, swollen, bloodshot, scarred. One popped off a good length he explained to me... Luckily, he said for whatever reason I decided to wear my helmet today so it could have been an awful lot worse.
I know it's not my head and it's each individuals choice but if one of my team mates gets hurt it's not nice is it. Yes I could say, well it was their choice but it's still not a nice thing to happen. Like if somebody gets hurt in a car accident when not wearing a seatbelt again it's their choice but the outcome is still grim.
Another angle on this... I hate bowling at people with no helmet, again not my head, but for me to deal with if an accident happens and they get injured off my bowling... If you want a game of high octane cricket go and bat with no pads on, yes you'll come away with some bruises but it might improve your footwork!!!!