Although the federal government may use its power of the purse to encourage states to adopt certain criminal laws, the federal government is limited in its ability to directly influence state policy by the Tenth Amendment, which prevents the federal government from directing states to enact specific legislation, or requiring state officials to enforce federal law. As such, the fact that the federal government has criminalized conduct does not mean that the state, in turn, must also criminalize or prosecute that same conduct.
Court decisions have been clear on this point, add Garvey and Yeh, emphasizing that "under both Tenth Amendment and preemption principles, federal and state courts have previously held that a state’s decision to simply permit what the federal government prohibits does not create a 'positive conflict' with federal law."

This doesn't mean that engagers in activities formally forbidden by D.C. are home-free, however, since "[t]he federal government remains free to expend its own resources to implement and enforce its own law, regardless of whether the state chooses to criminalize that same conduct." The authors acknowledge, however, that most of the would-be enforcers work at the state and local level, leaving the feds a tad short-handed when it comes to busting growers, sellers and smokers of marijuana.

Thus far federalism takes the states, but perhaps not much farther. Washington and Colorado both envision bureaucratic regulatory and licensing regimes for marijuana, and that goes well beyond declining to enforce federal law and likely sets up an actual conflict with federal objectives. The states might be able to overcome such objections, however, continue Garvey and Yeh, by arguing that regulations limit access to marijuana or simply help to enforce compliance with state law. Whether that would fly would have to be determined in court.

Frankly, the position of the states would be much clearer, according to the CRS report, if they simply adopted a laissez faire regime and let the freak flag fly without government officials taking a regulatory role.

Less encouraging is Garvey's and Yeh's discussion of mechanisms available to a manpower-short federal government in the absence of cooperation by local police and state courts.
However, even if the probability of becoming the subject of a federal criminal prosecution for a violation of the [Controlled Substances Act] appears remote, there does exist a number of other consequences under federal law that are triggered by the mere use of marijuana, even absent an arrest or conviction. Perhaps most prominent among these concerns is the possibility that marijuana users may lose their ability to purchase and possess a firearm, be barred from living in public housing, or find themselves subject to employment consequences in the workplace.
That's in addition to asset forfeiture and civil lawsuits. Why yes, we have handed the federal government an awful lot of clubs with which to beat us into submission. But remember that the states don't have to help the feds identify marijuana users — eliminating regulatory and licensing bureacracies would be a nice step toward making such identification even harder. And, if the states can opt out of enforcing marijuana laws, it seems logical that they could do the same for firearms laws, too, as well as some of the other legal bludgeons D.C. might wield.

In short, voters in Colorado and Washington haven't quite ended America's long and unpleasant experience with marijuana prohibition. They have, however, deprived the federal government of most of its enforcement mechanisms by taking their state officials out of the game. That doesn't render federal drug warriors powerless, but it leaves them with many fewer options.