Suppose that groceries were supplied in the same way as K-12 education. Residents of each county would pay taxes on their properties. Nearly half of those tax revenues would then be spent by government officials to build and operate supermarkets. Each family would be assigned to a particular supermarket according to its home address. And each family would get its weekly allotment of groceries—"for free"—from its neighborhood public supermarket.

No family would be permitted to get groceries from a public supermarket outside of its district. Fortunately, though, thanks to a Supreme Court decision, families would be free to shop at private supermarkets that charge directly for the groceries they offer. Private-supermarket families, however, would receive no reductions in their property taxes.

Of course, the quality of public supermarkets would play a major role in families' choices about where to live. Real-estate agents and chambers of commerce in prosperous neighborhoods would brag about the high quality of public supermarkets to which families in their cities and towns are assigned.

Being largely protected from consumer choice, almost all public supermarkets would be worse than private ones. In poor counties the quality of public supermarkets would be downright abysmal. Poor people—entitled in principle to excellent supermarkets—would in fact suffer unusually poor supermarket quality.

How could it be otherwise? Public supermarkets would have captive customers and revenues supplied not by customers but by the government. Of course they wouldn't organize themselves efficiently to meet customers' demands.

Responding to these failures, thoughtful souls would call for "supermarket choice" fueled by vouchers or tax credits. Those calls would be vigorously opposed by public-supermarket administrators and workers.

Opponents of supermarket choice would accuse its proponents of demonizing supermarket workers (who, after all, have no control over their customers' poor eating habits at home). Advocates of choice would also be accused of trying to deny ordinary families the food needed for survival. Such choice, it would be alleged, would drain precious resources from public supermarkets whose poor performance testifies to their overwhelming need for more public funds.