Mr Davis' family have fought for a prosecution in relation to his death, with his mother Victoria Davis telling the coroner's inquest in December that Department of Corrections staff had failed her son in their duty of care.
She told the court she was concerned that some prison staff had been "so indifferent to his wellbeing" that he had "unnecessarily died".
Mr Davis had drugs inside him when he went into the Otago Corrections Facility as a remand prisoner in February 2011.
Although Corrections staff knew that from an intercepted phone call, he was placed alone in an at-risk cell and did not receive medical help. He died in the cell two days later.
Roger Brooking, alcohol and drug counsellor and prison reformer, said the decision not to prosecute was disappointing but typical of an inherent systemic bias.
Mr Brooking said Detective Senior Sergeant Colin Blackie, initially in charge of the investigation, "made it very clear" he would have launched prosecutions.
But Mr Brooking said Mr Blackie was taken off the case and his replacement, Detective Inspector Steve McGregor, was "higher up the food chain" and that changed the investigation dynamics.
Mr Brooking said he would support Mrs Davis if she decided to launch a private prosecution.
"In the last ten years approximately 90 prisoners have died unnatural deaths and the police have never prosecuted anybody," Mr Brooking said. "This is just part of a pattern.
"I can't see anything changing unless or until somebody is actually prosecuted."
Mr Brooking said Corrections repeatedly ignored coronial recommendations, or responded with hollow promises.