They have to keep everything contained within the state so that the Commerce Clause cannot be cited as the Constitutional authority for the feds to be involved.
Not so:
Where necessary to make a regulation of interstate commerce effective, Congress may regulate even those intrastate activities that do not themselves substantially affect interstate commerce.
J. Scalia, Raich
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Drug warriors cheered at the time.
But in Gonzales v. Raich (the medical marijuana case), the Supreme Court held (with Scalia concurring) that marijuana grown and consumed solely in California, and never sold anywhere to anyone, could be prohibited under the Interstate Commerce Clause.