Why would a "justice system" refuse to test DNA evidence?

Conor Friedersdorf asks,

What Kind of 'Justice System' Refuses to Test DNA Evidence?

He goes on to tell the story of Sharon Snyder, who was fired for helping to exonerate an innocent prisoner.

Robert Nelson, 49, sought DNA testing to overturn his rape conviction in 2009 and again in 2011, but Judge David Byrn denied both requests because he hadn't crafted the motion properly. "After the second motion failed in late October 2011, Snyder gave Nelson's sister, Sea Dunnell, a copy of a motion filed in a different case in which the judge sustained a DNA request," AP reports. Using that public document as a guide, Nelson finally won the right to have the DNA evidence tested on February 22, 2012. Last month, that DNA test proved him innocent.

Why deny any inmate a DNA test that could definitively prove that he was innocent, especially in a country where DNA evidence has led to the post-conviction exoneration of 310 people? On average, those wrongfully incarcerated men served 13.6 years in prison.

When an inmate is exonerated by DNA, the most important consequence is the release of an innocent man or woman from prison, but that isn't all that happens. Taxpayers need no longer pay for room and board. Family members and friends stop suffering an absence. And in roughly half of DNA exonerations to date, the true perpetrator of the crime has been identified by the evidence. Everyone wins, save for the police, prosecutor, judge, and jury who put the innocent man away. They ought to admit their mistakes. Many do. But sometimes they keep fighting.