Why would the government make a law against "thirds"?

The government is controlling population, and only allowing "thirds" if the first two children show enormous potential. Think about China today..... they have a one-child rule that applies to most of the population and is only relaxed in the instance where both parent are "only" children. In Ender's Game, the government and most of its citizens believe the two-child rule to be best. Parents of promising children may apply for a third.