When Owen Beck was 17, doctors amputated his right leg to stop the spread of bone cancer. His parents, desperate to find a drug that would relieve their son's excruciating phantom limb pain, brought him to Charlie Lynch's medical marijuana dispensary in Morro Bay, Calif., carrying a recommendation from a Stanford University oncologist. The marijuana not only eased the pain but also alleviated the nausea caused by chemotherapy.

Called to testify as a character witness in Lynch's federal marijuana trial, Beck did not get far. When he mentioned his cancer, U.S. District Judge George Wu cut him off and sent him packing. Wu decreed there would be no talk of the symptoms marijuana relieves, no references to California's recognition of marijuana as a medicine, no mention even of the phrase "medical marijuana" in front of the jury.

In short, there would be no explanation of how Lynch came to operate what prosecutors called a "marijuana store" in downtown Morro Bay for a year, openly serving more than 2,000 customers. Under federal law, which forbids marijuana use for any purpose, all that was irrelevant. So it's hardly surprising that Lynch was convicted last week of five marijuana-related offenses that carry penalties of five to 85 years in prison.

Nor is it surprising that so many self-described conservatives, including Republican presidential candidate John McCain, support the prosecution of people like Charlie Lynch, abandoning their avowed federalist principles because of blind hostility toward a plant they associate with draft-dodging, flag-burning hippies. It's not surprising, but it's shameful.

The U.S. Drug Enforcement Administration (DEA) has raided more than 60 medical marijuana dispensaries in the last two years. Because the deck is stacked against them, dispensary operators facing federal drug charges typically plead guilty.

Lynch instead gambled on a defense known as entrapment by estoppel, which occurs when someone is arrested for actions the government assured him were legal. Before he opened Central Coast Compassionate Caregivers in 2006, Lynch called the DEA to ask about his legal exposure. He says an agent told him he should consult with state and local authorities, which he took to mean he could avoid trouble as long as he complied with state and local law.

It's not hard to see why Lynch believed he was operating a legitimate business. He had the blessing of the Morro Bay Chamber of Commerce and the city council; local officials, including Morro Bay's mayor, posed for pictures at the dispensary's opening; and neither his neighbors nor the city police objected.

At Lynch's trial the DEA denied giving him any sort of green light, or even a yellow one. But the response he says he got from the agency is the response he should have gotten, because under the U.S. Constitution the medical use of marijuana is a local matter.

At one time John McCain seemed to acknowledge as much. In April 2007 he said, "I will let states decide that issue." But he quickly abandoned that position, and this year he said he'd continue the DEA's medical marijuana raids, declaring, "It is a national issue and not a [state] issue." By contrast, McCain's Democratic opponent, Barack Obama, has promised to stop the raids.

McCain's medical marijuana position contradicts his professed allegiance to federalism. "The federal government was intended to have limited scope," he says on his website, vowing to appoint judges who "respect the proper role of local and state governments."

That commitment is inconsistent with reading Congress's power to regulate interstate commerce broadly enough to cover homegrown medical marijuana, as the Supreme Court did in 2005. "If Congress can regulate this under the Commerce Clause," Justice Clarence Thomas noted in his dissent, "it can regulate virtually anything -- and the federal government is no longer one of limited and enumerated powers."

By supporting the Bush administration's medical marijuana policy, McCain is renouncing such concerns. Worse, his promise to flout the Constitution probably will enhance his appeal among conservatives.