The Question I'd Have Asked at the Obamacare Hearings

For many years, libertarians like those at the Cato Institute have advocated replacing Social Security with a mandate on all citizens to save for their retirement in a privately managed account.

Question: If it's unconstitutional (as the challengers to the Affordable Care Act now argue) for government to require citizens to buy health insurance coverage from a private provider, how can it possibly be constitutional for government to require citizens to buy a retirement annuity from a private provider?

The logic of the challenger's case is that the only constitutionally permissible way to provide for social needs is through a big government tax-and-redistribution program.

If the healthcare mandate is unconstitutional, how can compulsory private retirement accounts be permissible?