Yes, the minimum arrow weight is regulated. Per the current rules, you can shoot any weight broadhead arrow you want as long as it is 500 or more grains. It simplifies the certification of the equipment and represents a realistic average hunting weight arrow.
A 10 grains per pound requirement could complicate the process, especially when a competitor learns that their bow gained draw weight in the dry Salt Flats climate. A fixed grains per pound also makes it very difficult for a person with a short draw length to be competitive in the unlimited draw weight category. With the 500 grain minimum, a short draw person can make up for their lack of draw length with higher draw weights.
Alan