Same reason they wear this for a helmet:
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Those will keep your brains in nice condition even after they separate from your body.
Why? to answer this question, i did yahoo answer search.
"The reason I only wear a half helmet (other than a helmet is required in CA) is because I know my Harley is only about half as fast a sport bike. Therefore I know I will never hit the ground as hard as those losers would." - i hit ground so much softer then you, LOSER! Follow me and grow some fat roles on your head!
"The half helmt has it's roots in the mandatory helmet laws, which the vast majority of us oppose. Don't get me wrong, we are not all opposed to helmets, we are only opposed to being told we have to. There is a Motorcycle Rights Organization called ABATE, American Brotherhood Against Totalitarian Enactments. It was started in 1973 by Easy Rider Magazine. There are now ABATE groups in each of the 50 states, and many chapters in each state. Now a days we deal wtih lots more than just the helmet issue but that is where we got our roots." Damn the man for trying to protect the stupid.
"Full face helmets are dangerous." Sure are...
"Because they have faith that their loud pipes will save their lives." Yep.