In Britain, every prisoner when brought into the system (and to an extent even prior to being sentenced) must work upon their sentence plan.
The sentence plan is absolutely crucial and cannot be underestimated.
During the early report write ups, the offence is examined, along with the main reasons for the occurrance.
The original offence might well be drug related, but at the back of this, the reasons could be poor education, very often it is low level mental and sociological problems.
I believe I have posted figure before on the astonishingly high numbers of mental illness and social problems, along with the almost unbelievably bad education levels of most prisoners.
The idea is that the vast majority of offenders wil be released at some point, its very desirable that when released, the offender has addressed the issues that led them into offending, or what failed on their previous time in prison that led them to reoffend (with up tp 80% reoffending rates this is a big issue)
Its not just a case of admitting guilt, it is a recognition of what needs to be worked on to prevent more crime.
Admitting guilt is simply words, nothing more, it is realising that, for example, the drug taking led to the burglary, but the drug taking itself was because the offender was excluded from mainstream society by their unemployability, which could in turn lead to the conclusion that the offender has extremely poor education, perhaps becuase they were excluded from school because they were disruptive.
You can go a long way into an offenders history, with multiple factors and influences that have drawn them into crime.
Other matters could be emotional immaturity, the offender resorted to violence because they simply were not mature enough to understand that there are many ways of dealing with conflict, so part of the work they may have to do is to develop the social skills necassary to see the other options (even when provoked)
To even begin dealing with these matters, and the necessary eduction/training/therapy, the whole must be supported by some sort of motivation not to offend again, or at the least, reduce the level of harm, perhaps by changing the drug of choice.
If a prisoner does not accept their responsibility for their role in their offence, and perhaps any flaws in their background and character, its is not realistically possible to engage in a meaningful attempt to rehabilitate.
If a prisoner cannot accept the real reasons for their offence, and if they do not truly accept their guilt, then they will not be able to work on their problems, so it makes sense to keep them in prison as long as possible to keep the public safe.