So why was he at an open prison?

There are times I simply cannot grasp the idiocy that goes on in our justice system, from ridiculous sentencing to where they do put the prisoners when convicted.
BBC.

A prisoner who was jailed for making threats to kill and possessing an offensive weapon has absconded from a Teesside open prison.
Bernard Sharkey, 34, was serving an indeterminate sentence at HMP Kirklevington Grange near Yarm.
He was last seen carrying out community work at Newham Grange Farm in Coulby Newham at 12:30 BST on Thursday.
Cleveland Police warned members of the public not to approach Sharkey but to call 999 immediately.
He was serving an indeterminate sentence for threats to kill, affray and possession of an offensive weapon.
Officers said he may pose a risk to a "known individual", who had been contacted, but not to the wider public.

Does it not strike anyone else as odd that a man convicted of making threats and possessing an offensive weapon was being held at an open prison? You know, the type that only those who are trusted not to run or who are being rehabilitated at the end of their sentences are?
We keep hearing reports that the prison system is too crowded, that sentencing guidelines are being changed to allow lesser sentencing (and not so much about building new prisons) and there are sentences which the accused does go to prison which could warrant a non custodial sentence. But it does strike me that we keep the dangerous ones at least behind lock and key.
Hopefully Sharkey will be apprehended without incident, we really don't need another Raoul Moat on the loose. But it is clear that lessons are not being learned and that dangerous prisoners are not being monitored anything like closely enough.