Counterfeiting is one of the crimes mentioned in the Constitution; it is explicitly put under the control of Congress and therefore properly a federal crime — no such power exists for drugs.
You and that other guy who pointed this out are missing the forest by focusing on a tree. The point is that the consequences of some activities do not remain within the state.
So let's say they aren't counterfeiting money, but are instead counterfeiting movies or clothing, or whatever. Would the consequences stay within the state?
Pot is an activity that won't remain in a state. If one state has it, it will encourage people in other states to engage in it as well. (as the lawsuits are even now alleging) It will grow and spread as drugs invariably do if given a host population.