Signing contracts under duress - Obamacare loses?

I love that the Institute for Justice has filed an amicus brief to the Supreme Court pointing out that contracts signed under duress have never been legal. Forcing people to buy an insurance contract they may not wish to buy, and forcing insurance companies to insure anyone, no matter what pre-existing condition they may have, is putting both parties in the contract under duress by the government. Anthony Kennedy, you're the swing vote - are you listening? How can the Supremes uphold Obamacare in light of this salient fact?