Rehabilitation or Protection?

In a report about the accused Norwegian right wing terrorist Anders Breivik. the LATimes reported:

-- If declared sane, Breivik could face a maximum 21-year prison sentence or an alternate custody arrangement that would keep him locked up as long as he is considered a menace to society. If found insane, he would be committed to psychiatric care for as long as he's considered ill. --

I find this especially interesting, as I have long suggested that we need to rethink our attitudes in regard to the purpose of the criminal justice system.  I do not oppose the idea of rehabilitation, but feel that the idea of clearing a dangerous violator of his crime by "paying his debt to society" is absurd.  Depending upon the seriousness of the crime and of theoretical threat to society. I contend that if an individual has committed a genuinely damaging act, and if that person constitutes a  continued danger, they should not be released from custody. If rehabilitation can be reasonably shown to have occurred, then the danger is eImitated and the offender can be released.  
However, the idea of releasing an individual who constitutes a serious danger to his fellow citizens when he is currently safely held in custody, seems to me to be irrational.  I prefer the Norwegian "alternate custody arrangement that would keep him locked up as long as he is considered a menace to society."

Remember that this only applies to dangerous individuals convicted of serious crimes. It should not be applied except in the most serious is cases.