Although this may sound like a humorous rhetorical question, it actually does have a real answer. Why would a kamikaze pilot wear a helmet inside of his plane if his end goal was to crash anyway?
The truth is, all pilots wear helmets, and no matter if they were trying to crash or not, a helmet really would not have helped that much. The speed of the plane upon impact would have been too great for the helmet to really affect anything, however, there is still a legitimate reason for wearing a helmet. In the World War II era, pilots (including Kamikazes) would sometimes fly with their cockpits open. This would have helped them get a better view of things, and for non-kamikaze pilots it would have made ejection easier in case of a crash. Their helmet would have just been a padded piece of leather to keep their head from getting too cold in the wind. It would have also helped as they dodged gunfire on their final nosedive towards an enemy ship. Such quick turns would have bounced their heads all around and their helmet would have protected them form head injury before crashing. Once they crashed, however, their helmets were pretty much useless.