The Government, as we have experienced many, many, times over the last 30 years, can be a law unto themselves, and can do just about whatever they want, ( even ignoring Judicial rebuke ), so canceling these carriers is feasible, plus it will additionally save all the accrued costs of manning and operating them for how ever many years they are planned to be in service. If the private companies involved want to complete them, they can find another buyer, it is, after all, a free market economy.