New Immigration Policy Announced

The Department of Homeland Security today announced a new policy on immigration enforcement.  In summary the policy will allow the exercise of discretion by the Executive Branch to defer action, on a specific group of individuals who might otherwise be subject to deportation or removal.  As the announcement says, this group includes individuals who:

1.) Came to the United States under the age of sixteen; 2.) Have continuously resided in the United States for a least five years preceding the date of this memorandum and are present in the United States on the date of this memorandum; 3.) Are currently in school, have graduated from high school, have obtained a general education development certificate, or are honorably discharged veterans of the Coast Guard or Armed Forces of the United States; 4.) Have not been convicted of a felony offense, a significant misdemeanor offense, multiple misdemeanor offenses, or otherwise pose a threat to national security or public safety; 5.) Are not above the age of thirty.

My initial instinct is that this is an effort to adopt by Executive authority and the exercise of prosecutorial discretion the provisions of the DREAM Act which, to date, have failed of passage in Congress. The policy raises an interesting legal question -- at what point does the exercise of prosecutorial discretion become an inappropriate nullification of law.  The question is not, of course, unique to the immigration context.  Imagine, if you will, an announcement by the Executive Branch that it would no longer prosecute any narcotics offenses.   I'm sure there is a line somewhere out there -- but I have no idea where it is.