The author concludes that existing laws have legitimacy simply because they are the laws. This is based on a subsidiary conclusion that, if the purpose of laws is not to contribute to people’s happiness, then we don’t have a basis for evaluating existing laws. This sub-conclusion is based on the premise that if the purpose of laws is to contribute to people’s happiness, then we have a basis for criticizing laws.

The author confuses a sufficient condition for having a basis for criticizing existing laws with a necessary condition. Although we know that if the purpose of laws is to contribute to happiness, then we have a basis, that doesn’t imply that if the purpose of laws is not to contribute to happiness, that we no longer have a basis to criticize laws. So the author’s jump to the sub-conclusion is flawed.