People do all sorts of crazy things. Maybe they're "just out for a short
ride" and planning on doing a long one later, so they have the helmet
along but don't want to mess with it just yet. Maybe they're on the way to
a meeting and are keeping their hair nice, but will wear it on the way
home. (Neither's a convincing argument to me, but they are reasons :) )
For those who need helmets and can't afford, I think HELL's still active:
http://www.hellmets.org/
-mook.