I think Matthew’s legal and constitutional concerns about the Obama Administration’s new policy concerning medical marijuana to be misplaced.
First, it is not at all uncommon for the legality of conduct under federal law to be dependent upon state law. To take the most obvious example, under 27 U.S.C. 122, it is against federal law to transport alcoholic beverages into a state in violation of state law. Therefore, under federal law, it is legal to something in one place but not another because of differences in state law. The purpose of such laws is to allow states to adopt laws that reflect local preferences while simultaneously protecting states from the spillover effects of their neighbors’ choices. So, in the case of alcohol, the post-prohibition policy was to say: State A can legalize alcohol if they like, but the federal government will not allow State A to become a source of illegal alcohol for State B, in which alcohol is still prohibited. This sort of law makes perfect sense within our federal system. Indeed, such federal laws are preferable to federal efforts to create uniformity on divisive questions of social policy.
Second, it is also not anomolous or particularly unusual for the federal government to announce a policy of declining to prosecute to the full extent possible under given laws or statutes. The federal government lacks the resources to pursue all acts prohibited by federal law, so choices must be made. In some cases, it makes sense to base those chocies on broader policy judgments than ad hoc, case-by-case determinations. This is particularly common in the regulatory context. A given regulatory statute may, on its face, apply to thousands of instances of a given conduct. But the relevant enforcement agency, seeking to focus its efforts where they wil do the most good, may announce a policy of focusing its efforts on those instances where there are real harms. So, for instance, in the environmental context, the EPA may announce it will prioritize regulatory violations that result in measurable amounts of environmental contamination, and generally ignore paperwork violations. This doesn’t make such conduct legal, as the agency in question inevitably reserves the right to prosecute any violation — as the Justice Department has here — but it does provide greater predictability, which is an important rule of law concern.