DNA evidence exonerates the accused so why do DA's continue to prosecute?

Hundreds of people in the United States have been cleared by DNA evidence over the last two decades, in some cases after confessing to crimes, often in great detail. Juveniles, researchers have found, are more likely to make false confessions. Four of the five teenagers who were convicted in the brutal 1989 rape of Trisha Meili, known as the Central Park jogger, for example, confessed to the rape but were later exonerated when DNA evidence confirmed another man’s involvement.

For most prosecutors, the presence of post-conviction DNA evidence is enough to prompt action. An examination of 194 DNA exonerations found that 88 percent of the prosecutors joined defense lawyers in moving to vacate the convictions. But in 12 percent of the cases, the prosecutors opposed the motions, and in 4 percent, they did so even after a DNA match to another suspect.

Brandon L. Garrett, a professor of law at the University of Virginia, who studied the exonerations last year, said that many of the cases in which prosecutors dispute the significance of DNA evidence involve defendants who initially confessed to the crimes.       
http://www.nytimes.com/2011/11/16/us/dna-evidence-of-innocence-rejected-by-some-prosecutors.html?_r=2&pagewanted=all?src=tp

Texas- DNA Exonerations Continue, but Not for One Man:
http://www.texastribune.org/texas-dept-criminal-justice/hank-skinner/dna-tests-bring-freedom-not-skinner/


Morton Investigator, Medical Examiner Testimony Public:

The former sergeant who led the investigation that resulted in Michael Morton's wrongful conviction in 1987 told lawyers last week that then-District Attorney Ken Anderson wanted to see every report that investigators made as they tried to determine who brutally murdered Christine Morton. If there was information in the file about leads that could have indicated someone else was the killer, retired Williamson County Sheriff's Office Sgt. Don Wood indicated, Anderson would have had access to those details.

Wood provided testimony as part of an ongoing investigation into whether Williamson County officials intentionally withheld information that could have prevented Morton's conviction and life sentence. His statements indicate that Anderson, who is now a Williamson County district judge, would have known about clues that indicated Morton was not the killer. Wood's transcript was made public Friday, along with a statement from the medical examiner in the case refuting allegations that prosecutors made during trial about scientific evidence that proved Morton was the killer.
http://www.texastribune.org/texas-dept-criminal-justice/court-of-criminal-appeals/morton-investigator-medical-examiner-testimony-pub/