"The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people."
"The Amendment expressly declares the constitutional policy that Congress may not exercise power in a fashion that impairs the States' integrity.'' Fry v. United States, 421 U.S. 542, 547 n.7 (1975).
Whereas Medical Marijuana is regulated within the state's borders and does not cross other state lines, there is no "interstate commerce", making that argument moot.
As, with many other issues, you cannot choose which parts of the U.S. Constitution you want to follow. Allow the state governments/voters make decisions that affect only those within their borders, instead worry about securing the U.S.'s borders as a whole.