While in principle, presidents are not supposed to legislate, that power being reserved to the Congress under the Constitution, it has been recognized since the earliest days of the republic that the president's executive power necessitates some degree of de facto lawmaking.
Every statute unavoidably conveys some discretion.
From this fact inevitably flows a measure of presidential lawmaking via executive order. But clearly the power is not unlimited. When executive orders lack statutory support, they have been held by the courts to be without the force and effect of law. Executive orders may not supersede a statute or override contradictory congressional expressions. An even better authority on this issue is William Howard Taft, who served both as president of the United States and later Chief Justice of the Supreme Court.