This is more of a question of how this came about more than anything.
At WPO, we had two guys come that had full metal helmets. It was brought to my attention and I had to tell them to take it off or be unable to get on the field (They left, sadly )
How did this rule come into place? I assume something happened way back in the day and someone got REALLY hurt and it's mostly an overall safety thing... but i'm more curious about why this rule came to be.