During last night's class, we had a terrific discussion that weaved several different constitutional principles together. The two principal questions spurring the discussion were these:
1. How is it that California's Compassionate Use Act (or, more clearly, Prop 19 if it passes) does not "frustrate the purpose" of the federal Controlled Substances Act?
2. How is the California marijuana example (where state law is not preempted) distinguishable from Arizona's S.B. 1070, which essentially creates state offenses that are tied to the violation of federal law, and thus indirectly permits state officers to enforce federal immigration law?
These are complicated issues, and one blog post cannot comprehensively address all of the subtleties. But I think the essential point is this: state laws that permit certain behavior generally can never be preempted. For all they do, at a basic level, is is provide that the state will not regulate the activity in question (or a specific subset of that activity). The easiest way to see why this must be so is that, if a permissive state law were preempted by a federal statute, then the federal statute would effectively be commanding the state instead to regulate that activity. And we know that, under the anti-commandeering principle, Congress cannot require states affirmatively to regulate according to the federal government's instructions. Thus, so long as the state law is merely permitting something to occur (whether that permission is characterized as "action" or "inaction"), it cannot be preempted.
(No doubt, if Prop 19 passes, it will frustrate the purposes of federal law in a colloquial sense. But it will not in a legal sense, due to the implications of such an understanding outlined above.)
In contrast, state laws that forbid certain activity, or penalize or sanction that activity, are clearly subject to preemption. These are the laws that can frustrate the purpose of federal law. For example, the Illinois common law rule concerning unreasonably dangerous consumer products was potentially subject to preemption because it imposes liability on product manufacturers (in certain circumstances).
And the same is true of Arizona's SB 1070. Among other things, it (1) requires that all persons who have been arrested have their immigration status verified prior to release, (2) requires police officers to make a reasonable attempt to determine an individual's immigration status during any lawful stop, (3) makes it a misdemeanor for a person unlawfully present in United States to apply for work, and (4) permits police officers to make warrantless arrests when the officer has probable cause to believe that the suspect has committed an offense that renders the suspect removable. All of these are limitations imposed on activity, or the authorization to impose such limitations. Declaring these provisions preempted would not force Arizona to do anything--other than to stand down its enforcement machinery. (It need not even actually repeal its law; it would just be enjoined from enforcing it.)
So the real difference is between state laws that permit (like California's more permissive marijuana policy) and those that forbid, constrain, or sanction. The latter are subject to preemption, while the former are not.