Ooh, I know. Call on me!
It is the bastardized Wickard Commerce Clause and its progeny. This has allowed fedgov to control health care, education, the environment and a slew of other areas never intended. Here's a real world example of how Drug War Whores in Raich expanded the Commerce Clause beyond what even the Ninth Circuit was willing to allow. =>
Not long after the decision in Raich, the Court vacated a lower court decision in United States v. Stewart and remanded it to the court of appeals for reconsideration in light of Raich. In Stewart, the Ninth Circuit had held that Congress lacked the Commerce Clause power to criminalize the possession of homemade machine guns.
http://en.wikipedia.org/wiki/Gonzales_v._Raich
Given that the Wickard court was 8 Roosevelt appointees and one liberal non-Roosevelt appointee, it is safe to say it does exactly what they intended that it should do, but the Commerce clause is a poor constitutional argument for making drugs illegal. The Defense clause is far more appropriate.
That they use commerce and Wickard to assert authority over drugs is just a matter of convenience. Because the liberal courts say they can use it as justification they do so, but it isn't the best fit.