In short, the law converts insurers into private tax collectors, collecting mandatory premiums from the healthy to pay for politically popular changes in the insurance laws.

Drawing on copious statistics, the judges show that today’s health-care “free riders” are largely illegal immigrants (who are exempt from the mandate) and low-income Americans (who will get coverage under the law’s vast expansion of Medicaid). In other words, the mandate doesn’t prevent free riding -- it enforces it.

In imposing the mandate, wrote the judges, “the Congress sought to mitigate” the costs of popular reforms like covering pre-existing conditions and removing lifetime caps on payouts “by compelling healthy Americans outside the insurance market to enter the private insurance market and buy insurers’ products.”

That is, it forces people to enter the insurance market -- and forcing people to enter into commerce is different from regulating them once they voluntarily engage in it. Only the latter is constitutional.