Stupid helmet question

I was browsing a newsgroup just now and saw a gentleman flying a P-51
Mustang with what appears to be just a common ball cap. I got to
wondering if he is that secure in his abilities and crash worthiness
of the air frame to simply wear a ball cap?

As a motorcyclist I feel very strange if I'm not wearing the very best
helmet that I can afford. Why would a pilot operate a high performance
aircraft with anything less?

I know that a high dollar helmet and other protective gear has no
guarantees whether it's a car, motorcycle, pony or aircraft.

Am I missing something? Is the pilot simply exercising his freedom to
take his chances? (The law won't let me do that in some states.)