and they have wide latitude in doing so
There may be some latitude, but not 'wide latitude'... to quote the ruling in the recent Yucca mountain decision against the NRC:
Our analysis begins with settled, bedrock principles of constitutional law. Under Article II of the Constitution and relevant Supreme Court precedents, the President must follow statutory mandates so long as there is appropriated money available and the President has no constitutional objection to the statute. So, too, the President must abide by statutory prohibitions unless the President has a constitutional objection to the prohibition. If the President has a constitutional objection to a statutory mandate or prohibition, the President may decline to follow the law unless and until a final Court order dictates otherwise. But the President may not decline to follow a statutory mandate or prohibition simply because of policy objections. Of course, if Congress appropriates no money for a statutorily mandated program, the Executive obviously cannot move forward. But absent a lack of funds or a claim of unconstitutionality that has not been rejected by final Court order, the Executive must abide by statutory mandates and prohibitions.
It goes on from there quite a bit, but then this is nothing new, the President has a long history of playing "catch me if you can" with policies and selective enforcement.
An interesting and informative quote; thanks for sharing it. However, in this specific case I'm not sure it applies, as Obama is basically saying he won't challenge state laws on the issue. Unless there is precedent saying that the executive branch must enforce federal law over state law (which there very well may be, IANAL), I don't think it's relevant.
As to the act itself, not whether it oversteps his authority, I am rather supportive. I do not smoke pot, and I feel anyone who does so is an idiot wasting his life, but, on the other hand, I feel the same about alcohol, and just take a look a prohibition to see how well that turns out. Any move to reduce government expense (and, indeed, generate income by taxing the stuff), while reducing overcrowding in prisons, and simultaneously reducing the wealth of criminal organizations, is in my mind a good thing.
However, if these actions do in fact overstep presidential authority, then I want to know exactly how and why, so that I can argue towards curbing them. Governmental power creep, however well-intentioned (as it almost always is), must be fought at every opportunity. Vigilance is the price of liberty