This comes into play in the event of a contentious hard fork. Since the miners send their MAC, the parent can look up the customer from sales records. If that c

This comes into play in the event of a contentious hard fork. Since the miners send their MAC, the parent can look up the customer from sales records. If that customer is against the parent's position in the hard fork, they turn off their machines. The "enemy" fork loses massive hash power at a critical moment.