There is a valid and constitutional argument to be made that the president may refrain from defending and enforcing laws that he believes are palpably and demonstrably unconstitutional. These arguments go back to Thomas Jefferson, who refused to defend or enforce the Alien and Sedition Acts because, by punishing speech, they directly contradicted the First Amendment. Jefferson argued that when a law contradicts the Constitution, the law must give way because the Constitution is the supreme law of the land and all other laws are inferior and must conform to it. This argument is itself now universally accepted jurisprudence -- except by President Obama, who recently and inexplicably questioned the jurisdiction of the Supreme Court to invalidate the Affordable Health Care Act on the basis that it is unconstitutional.
Nevertheless, there is no intellectually honest argument to be made that the president can pick and choose which laws to enforce based on his personal preferences. And it is a profound violation of the Constitution for the president to engage in rewriting the laws. That's what he has done here. He has rewritten federal law.Only Congress can lay down specifics such as in order to avoid deportation and qualify for a two-year work visa, one must have entered the U.S. prior to age 16 and possess a valid American high school diploma or be a military veteran, as the president now requires. By altering the law in this manner -- by constructing the requirements the government will impose -- the president has violated his oath to enforce the laws as they are written. His second responsibility in the Constitution (the first is to defend the Constitution) is to enforce federal laws as Congress has written them -- hence the employment of the word "faithfully" in the Constitution -- not as he wishes them to be.
Congress should have enacted years ago what the president is now doing on his own, because it is unjust to punish children for the behavior of their parents, and it is unjust to restrict freedom based on the place of birth. But this can be remedied only by Congress. If the president can rewrite federal laws that he doesn't like, there is no limit to his power. Then, he will not be a president. He will be a king.