Ken and Alison Orchard said they were not surprised at the decision to end disciplinary hearings against two sergeants and two constables.
They hit out at Devon and Cornwall Police for its "incompetence, negligence and sloppy practices" in relation to the death of their son Thomas.
"Strangely, we are not surprised, and yet deeply shocked," they said.
"Despite their being charged with behaviour that could amount to gross misconduct, no further action will be taken against four police officers whose actions, together with others, in our opinion, directly led to Thomas' death.
"And this is because of, what we believe to be, incompetence, negligence and sloppy practices from the very people and processes that were meant to protect our son.
"We are not surprised because, in our assessment, we have witnessed it again and again over the past seven years.
"We are not surprised because, to be honest, we never really had any faith in this tribunal, believing it to be an investigation held - only reluctantly after direction from the IOPC - by the police, for the police.
"However, we also remain deeply shocked. As a family we used to believe in the system; we believed that if something bad happened, justice would be served."But no-one and no process that we have witnessed to date has fully explored - openly, honestly and constructively - the catastrophic failings surrounding Thomas' death.
"We feel let down and have been failed beyond belief.
"The saddest thing for us is that Thomas' death was in vain; worse than that, it seems to have reinforced the notion that the police can behave in ways that we see to have been grossly irresponsible, negligent and reckless... and get away with it."
The 32-year-old died in hospital seven days after being arrested and taken to Heavitree Road police station in Exeter, Devon, in October 2012 when suffering a mental health crisis.
During his detention, the church caretaker, who had mental health issues, was restrained and an emergency response belt (ERB) was placed across his face for five minutes and two seconds to prevent spitting or biting.
Misconduct proceedings were held against Sergeant Jan Kingshott, who was the custody sergeant, and Sergeant Alexander Kennedy, Pc Rob Dodd, and Pc Mark Nagle who were involved in Mr Orchard's detention and restraint.
The panel, which was chaired by Assistant Chief Constable Ben Snuggs, of Hampshire Police, concluded the four officers could not have a fair hearing after hearing preliminary legal argument.
In a landmark conviction in 2018, the office of the chief constable of Devon and Cornwall Police pleaded guilty to breaches under the Health and Safety at Work Act.
A judge at Bristol Crown Court ruled he could not be sure that the ERB - a tough webbing belt designed to restrain limbs - was a contributory factor in Mr Orchard's death.
He fined the force £234,500 and ordered it to pay £20,515 in prosecution costs.
Two years ago Sgt Kingshott, and civilian detention officers Simon Tansley, and Michael Marsden, were found not guilty of Mr Orchard's manslaughter by gross negligence.
Dismissing the gross misconduct allegations, Mr Snuggs said: "We are satisfied that there is irredeemable prejudice and it is impossible for the officers to have a fair hearing.
"The panel wants to be equally clear that this decision does not in any way seek to detract from the tragedy that is Thomas Orchard's death.
"As already identified, the case against these officers is for a distinct purpose and is separate from Thomas' death.
"We are very conscious of the separation of these issues in reaching our decision."
Andrew Berry, chairman of the Devon and Cornwall Police Federation, said: "Two criminal trials and a gross misconduct process have failed to evidence any wrongdoing on their part.
"Tragedies like the death of Thomas need to result in a legacy of learning. We hope the IOPC take note of this and carry out an internal investigation."
Shaun Sawyer, chief constable of Devon and Cornwall Police, said: "I accept the decision made by the independent panel.
"I understand the significant impact of this long-running matter on the officers, staff and their families, and of course the family of Thomas Orchard."
Sarah Green, regional director of the IOPC, added: "This has been a complex investigation in two parts, looking at both the actions and decisions of individual officers, and at Devon and Cornwall Police as a corporate body.
"We have also consistently said that police custody is not an appropriate place of safety for those detained under S136 of the Mental Health Act."
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