Theoretically the federal government can say that drugs can not be transported across state, tribal, or national borders which, in my opinion, is probably more practical since it won't interfere with an indian tribes ability to smoke payote for religioius ceremonies. They may not be allowed to carry it across their tribal borders but they can still trade it within their own borders if they wanted to. Also, some states may choose to ban drugs all together or regulate them in a unique way so a federal law that bans the transportation of drugs across state borders may help states better implement their own laws regarding the issue. Imagine if one state completely bans them but another lifts all restrictions. It is possible that the state that has no restrictions may become a 'distribution hub' into states where it is still illegal. A federal law banning the trade of drugs across state or national borders may better protect that state from those drugs or better implement its own policies.

“For the power given to Congress by the Constitution does not extend to the internal regulation of the commerce of a State, (that is to say of the commerce between citizen and citizen,) which remain exclusively with its own legislature; but to its external commerce only, that is to say, its commerce with another State, or with foreign nations, or with the Indian tribes.” –Thomas Jefferson, Jefferson’s Opinion on the Constitutionality of a National Bank : 1791. http://avalon.law.yale.edu/18th_century/bank-tj.a…

Also, note that before FDR's activist justices twisted the Commerce Clause in Congress's favor, the Court had clarified in United States v. Butler, and in terms of the 10th Amendment, that Congress has no constitutional authority to regulate agricultural production.

"From the accepted doctrine that the United States is a government of delegated powers, it follows that those not expressly granted, or reasonably to be implied from such as are conferred, are reserved to the states, or to the people. To forestall any suggestion to the contrary, the Tenth Amendment was adopted. The same proposition, otherwise stated, is that powers not granted are prohibited. None to regulate agricultural production is given, and therefore legislation by Congress for that purpose is forbidden." –United States v. Butler, 1936. http://supreme.justia.com/us/297/1/case.html