Americans who know Dr. Riyadh well think the charges against him are absurd.
So what's going on?
The doctor's case seems to be part of an effort by Iraq's Shiite-led government to marginalize Sunnis, including elected officials, and to monopolize power. But excluding Sunnis from the political system is a path that could return Iraq to chaos and sectarian warfare.
That's why the case of Dr. Riyadh symbolizes something much bigger than the fate of one man.
I first met Dr. Riyadh in 2003 through Col. Joe Rice, an Army reservist from Denver who went on to serve five tours in Iraq. Rice was advising Iraqis on setting up new local government institutions in Baghdad. He was working with Dr. Riyadh, who lived in Adhamiyah, a large neighborhood in Baghdad that became a hotbed of violent Sunni resistance to the American occupation.
After Saddam Hussein fell, most Sunnis boycotted elections, but Dr. Riyadh insisted that Sunnis had to participate in the new political process. He belonged to the Iraqi Islamic Party, a moderate Islamic political group that chose to compete by ballot rather than take up guns.
The doctor joined the local Adhamiyah Council (two of whose members were murdered), then ran successfully for Baghdad Provincial Council. (His risky political efforts were portrayed in a poignant 2005 documentary by Laura Poitras, My Country, My Country, that was nominated for an Academy Award.) He planned to run for reelection next year.
"Some people accuse me of working for the Americans," Dr. Riyadh told me during a 2004 visit to his Adhamiyah Free Clinic. "But I think I must take part in the political process because I want the people to have representation." The doctor was also protected by his reputation for treating the needy without charge.
When Rice invited the doctor and other council members to Denver in 2005 to observe local government, the doctor spoke out about abuses at Abu Ghraib prison, which infuriated U.S. authorities in Baghdad. But he also used his time to soak up information on the workings of the American political system.
"He wanted to learn about the powers and authorities of local government," Rice told me by phone this week, from Denver. "He was very interested in United Way and nonprofits and how they work." I asked Rice if he could imagine Dr. Riyadh being involved in violence and he replied forcefully, "No!"
So why has the doctor been brought before judges seven times, and each time returned to jail without a verdict (with the judge clearly under political pressure)? Why has he been deprived of the special food and medicine he needs for his diabetes and hypertension?
His plight reveals all too well the political mess the United States left behind in Iraq.
The U.S. invasion ended decades of control by the Sunni minority and propelled the Shiites into power. It also vastly increased the influence of Shiite Iran on Iraqi politics.
As U.S. troops drew down, Prime Minister Nouri al-Maliki's government feared a Sunni resurgence and began arresting Sunni parliamentarians, along with many Sunnis who had abandoned militancy and fought with American forces. Maliki also failed to honor a pledge to share power with the Sunni-dominated Iraqiya party, which won a narrow majority of seats in the last elections - despite intense efforts by Vice President Biden to broker a deal.
Maliki's fears appear to have been intensified by the war next door in Syria, where rebels from a Sunni majority are threatening to unseat the Alawite (Shiite) regime of Bashar al-Assad.
Yet the Iraqi leader's policies are extremely counterproductive. With Iraqi politics paralyzed, a few Sunnis extremists are once again trying to reignite a civil war by bombing Shiite civilians. Dragnet arrests of Sunnis will only ignite this fire.
If Sunnis are falsely accused and election results disrespected, or if they are prevented from running, Sunnis will feel that Iraq's system excludes them.
If democrats like Dr. Riyadh are left to languish in prison, it will signal that Iraq is heading down a dangerous road, with the United States unwilling or unable to prevent this. Human Rights Watch warned in February that Iraq was in danger of turning into "a budding police state."
Perhaps I'm being unfair to Prime Minister Maliki, who insists he wants to unify his country. If so, the release of Dr. Riyadh - and others like him - would be a brave and farsighted move in that direction.
When the doctor is next brought before a judge, on Oct. 18, his fate will signal which way Iraq is headed. I'll let my readers know the result.
E-mail Trudy Rubin