In my favorite pastime of motorcycling and scootering, many riders absolutely refuse to wear a helmet. Why? Cause it feels uncomfortable, and doesn't look macho. As if your brains spread across several lanes of highway does.
I know better, a helmet having saved my life in 1968. Less evolved and effective than the helmets we have today, I therefore still suffered serious head injury, the residuals having plagued me all my life to this very day, like poor memory, decline in intelligence, and eventually epilepsy.
Why do hockey players resist wearing their helmets? Why do boaters and jet-skiers refuse to wear lifejackets? Why do car drivers refuse to buckle-up?
Psychologists can fill books on the topic, and one theory can sound as convincing as the next. Bottom line is there's never any limit to the poor decisions people can make about themselves, from smoking to drinking to drugs, to other unsafe decisions.
Plus it's also a matter of degree. I think wearing a motorcycle helmet is a good choice. Others think going anywhere near a motorcycle in the first place is a bad one. But then driving a car is also risky. Life is a matter of risk assessment. In the case of the Red Sox player, to quote a line from one of the Indiana Jones movies, in my opinion:
"He choose... poorly."