The conflict has nothing to do with the federal government requiring the state government to outlaw marijuana. The conflict is, possessing, selling, or in any other way distributing marijuana is a federal offense. From the Controlled Substances Act [1]
"ยง 844. Penalties for simple possession.
"(a) Unlawful acts; penalties
"It shall be unlawful for any person knowingly or intentionally to possess a controlled substance unless such substance was obtained directly, or pursuant to a valid prescription or order, from a practitioner, while acting in the course of his professional practice, or except as otherwise authorized by this subchapter or subchapter II of this chapter. Any person who violates this subsection may be sentenced to a term of imprisonment of not more than 1 year, and shall be fined a minimum of $1,000, or both, except that if he commits such offense after a prior conviction under this subchapter or subchapter II of this chapter, or a prior conviction for any drug or narcotic offense chargeable under the law of any State, has become final, he shall be sentenced to a term of imprisonment for not less than 15 days but not more than 2 years, and shall be fined a minimum of $2,500, except, further, that if he commits such offense after two or more prior convictions under this subchapter or subchapter II of this chapter, or two or more prior convictions for any drug or narcotic offense chargeable under the law of any State, or a combination of two or more such offenses have become final, he shall be sentenced to a term of imprisonment for not less than 90 days but not more than 3 years, and shall be fined a minimum of $5,000. Notwithstanding the preceding sentence, a person convicted under this subsection for the possession of a mixture or substance which contains cocaine base shall be imprisoned not less than 5 years and not more than 20 years, and fined a minimum of $1,000, if the conviction is a first conviction under this subsection and the amount of the mixture or substance exceeds 5 grams, if the conviction is after a prior conviction for the possession of such a mixture or substance under this subsection becomes final and the amount of the mixture or substance exceeds 3 grams, or if the conviction is after 2 or more prior convictions for the possession of such a mixture or substance under this subsection become final and the amount of the mixture or substance exceeds 1 gram. The imposition or execution of a minimum sentence required to be imposed under this subsection shall not be suspended or deferred. Further, upon conviction, a person who violates this subsection shall be fined the reasonable costs of the investigation and prosecution of the offense, including the costs of prosecution of an offense as defined in sections 1918 and 1920 of title 28, except that this sentence shall not apply and a fine under this section need not be imposed if the court determines under the provision of title 18 that the defendant lacks the ability to pay."
"ยง 844. Penalties for simple possession.
"(a) Unlawful acts; penalties "It shall be unlawful for any person knowingly or intentionally to possess a controlled substance unless such substance was obtained directly, or pursuant to a valid prescription or order, from a practitioner, while acting in the course of his professional practice, or except as otherwise authorized by this subchapter or subchapter II of this chapter. Any person who violates this subsection may be sentenced to a term of imprisonment of not more than 1 year, and shall be fined a minimum of $1,000, or both, except that if he commits such offense after a prior conviction under this subchapter or subchapter II of this chapter, or a prior conviction for any drug or narcotic offense chargeable under the law of any State, has become final, he shall be sentenced to a term of imprisonment for not less than 15 days but not more than 2 years, and shall be fined a minimum of $2,500, except, further, that if he commits such offense after two or more prior convictions under this subchapter or subchapter II of this chapter, or two or more prior convictions for any drug or narcotic offense chargeable under the law of any State, or a combination of two or more such offenses have become final, he shall be sentenced to a term of imprisonment for not less than 90 days but not more than 3 years, and shall be fined a minimum of $5,000. Notwithstanding the preceding sentence, a person convicted under this subsection for the possession of a mixture or substance which contains cocaine base shall be imprisoned not less than 5 years and not more than 20 years, and fined a minimum of $1,000, if the conviction is a first conviction under this subsection and the amount of the mixture or substance exceeds 5 grams, if the conviction is after a prior conviction for the possession of such a mixture or substance under this subsection becomes final and the amount of the mixture or substance exceeds 3 grams, or if the conviction is after 2 or more prior convictions for the possession of such a mixture or substance under this subsection become final and the amount of the mixture or substance exceeds 1 gram. The imposition or execution of a minimum sentence required to be imposed under this subsection shall not be suspended or deferred. Further, upon conviction, a person who violates this subsection shall be fined the reasonable costs of the investigation and prosecution of the offense, including the costs of prosecution of an offense as defined in sections 1918 and 1920 of title 28, except that this sentence shall not apply and a fine under this section need not be imposed if the court determines under the provision of title 18 that the defendant lacks the ability to pay."
1. http://www.fda.gov/regulatoryinformation/legislation/ucm1487...