Because if it's empty then the cost is minimal (no catering, probably one or two less crew), and if it's full of op-ups then they are getting, in effect, J-class revenue for those seats which they obviously value more highly than miles. CX sells AsiaMiles at about US$0.01, so for them an F reward HKG-JFK RT at 220,000 miles is only US$2,200. There are probably passengers in Economy paying more than that!