I suspect that Obama did the right thing, but for the wrong reason. He has apparently directed his federal authorities to avoid prosecution of marijuana users whose activities are legal under their own state law. The right reason would be in recognition that crimes should be defined by the states, not the federal government. His reasons however are merely a shift in "prosecutorial priorities". In other words, he could always change his mind.
History teaches us that prior to the administration of George H.W. Bush, federal criminal laws punished truly national crimes like treason, bribery of federal officials and perjury in federal courts. But always to be expected, the federal government's voracious appetite to control people caused a significant leap forward after the 1980's and resulted in the federalization of otherwise state defined criminal conduct.
This sparked a tremendous growth in the whole bureaucracy which built up around this shift. More judges, prosecutors, and law enforcement agency resources were devoted to the investigation, prosecution, trial and ultimately imprisonment of those charged and convicted of federal crimes. The bite on our budget is more stinging today.
The federal government, justified this power grab primarily under the Commerce Clause of the US Constitution. Reasoning that crimes affect commerce, Congress stepped in where states were historically left to legislate on their own. The Constitutional justification for federalizing crimes is based upon a fiction and in fact one day may be dissolved. But the federal government's usurpation of the power of states to define crimes within their own borders has thus far survived for one reason only.
The federal government has been insulated by federal courts created by the Congress, populated with judges appointed by the President with the advice, consent and sometimes at the direction of members of Congress and as such the entire federal judicial system has been so far unwilling to call the federal criminal scheme for exactly what it is: utterly devoid of any real support under the Constitution.
Obama has taken a step in the right direction, but for the wrong reason. It is highly unlikely, given his propensity to strengthen the central federal government, that his actions signal any move back to the days when the sovereignty of states was fully acknowledged, and the limitations on the federal government were generally accepted. Odds are, he just doesn't think pot is a big deal.



