We (in the US) have this peculiar thing called "administrative law". The way it works sounds arcane, but it is really very simple: Congress makes the laws, the Federal agency with jurisdiction for that particular area makes the regulations which institute or apply that law, and then the regulations have the force of law, unless somebody sues the agency and the court finds that the regulation is contrary to the law it was supposedly promulgating. In this case, the Constitution, in Article I, Section 8, gives Congress the right to "establish a uniform rule of naturalization" and everything else follows from there...it is all somewhere in Title 8, US Code, but I would have to be getting caffeine intravenously to want to browse through there.