If the seat's going empty (according to the airline's load calculations), it's better to take some miles out of circulation for the seat than not. But why not charge 10 times the miles? Ask yourself who would pay that price. Most ff-hoarding individuals (value-oriented almost by definition) would find better value in flying 10 times in Y than one time in J or F. And the businesses that buy most of the J and F fares only very rarely use miles to purchase them.