Read it again boys.
The original question included the use of a fibreglass seat that was said in the first words of an attempt to answer to not be allowed.
Further details from others removed that confusion and may result in a rule that is more universally understood.
Just because you get it doesn't mean everybody does.
Imagine the shock for those with existing fibreglass seat forms that could have been left hanging.
Your butt is every bit as important as your head.
Most of us know a guy that came out from the bottom of a Drag Car back in the day, got lucky, and lived.
It was good to have "old whatshisname" around, however I did make my girls sit on his less dangerous side.