More Selective Law Enforcement From The Obama Administration...?

A few days ago Attorney General Eric Holder gave a speech at the American Bar Association whereby he announced that he is changing the way certain drug laws are enforced.  He said the administration is going to work with Congress to make changes to some laws governing crime and prosecution but in the meantime he is telling prosecutors not to follow the law as written but essentially make things up when it comes to non-violent offenders.

Holder's words were as follows:

"This is why I have today mandated a modification of the Justice Department’s charging policies so that certain low-level, nonviolent drug offenders who have no ties to large-scale organizations, gangs, or cartels will no longer be charged with offenses that impose draconian mandatory minimum sentences. They now will be charged with offenses for which the accompanying sentences are better suited to their individual conduct, rather than excessive prison terms more appropriate for violent criminals or drug kingpins."

My question is this - since when does the Attorney General of the United States have the authority to arbitrarily change the enforcement of laws to suit his agenda?  As Attorney General his duties are U.S. legal affairs and he is the chief law enforcement officer of the United States.  That says "law enforcement" officer, not "law making" officer.  

I've written before about this administration's propensity to enforce only laws they agree with.  A few years ago it was rumored that then Secretary of Homeland Security, Janet Napolitano, told Border Patrol agents to stop arresting illegal aliens entering the country to keep the numbers down, seemingly indicating that the borders were more secure.  We already know that just before the sequester took effect in March, ICE released thousands of illegals into the communities citing budget cuts that hadn't happened yet.  

And the Obama administration has delayed implementation of certain parts of the Affordable Care Act that seemed to be contrary to winning an election.  Again I ask - when did the President or the Attorney General get to select the laws (or parts thereof) they will enforce and those they will not enforce?  By now is this selective law enforcement not an impeachable offense?  But does anyone in the Congress have the courage to do it...?