The Congressional Research Service (CRS) is part of the Library of Congress, and it provides "policy and legal analysis to committees and Members of both the House and Senate, regardless of party affiliation." This month the CRS issued a report that says Colorado and Washington (where cannabis is legal, according to state laws) can't be coerced to enforcing federal cannabis laws. "While the federal government can ban what it wants," reports Reason, "the Tenth Amendment allows the states to opt out of participating in the law or assisting in enforcement in any way, leaving federal officials to do the heavy lifting themselves." From the report, State Legalization of Recreational Marijuana: Selected Legal Issues:
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.