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"mentally retarded" what is this, I don't even...
Here, something for you.Prison is not punishment, but rehabilitation. If you're putting people in prison to punish them, then you're doing it wrong.
Prison sentence is punishment, deterrent, preventative measure and rehabilitation all-in-one. But as ebbor says below, it isn't prison sentence, so I was misled by the article.
So first of, let me remove some misconceptions.
1) This person is not imprisoned. He's clinically insane, and thus has been interned for his own safety and that of others. Unfortunately, due to budgeting issues, prisons are considered proper psychiatric interning facilities. Interment takes until the person is no longer a threat to society.
2) This person is not responsible for his deeds. He did not get a life sentence, and in fact, was not prosecuted for his crimes. Once you're considered "Ontoerekeningsvatbaar", you're no longer considered responsible for any of the acts committed during said psychic episode. I think it's known as the insanity defense or something.
3) The Belgian governement is legally required to provide appropriate treatment.
Okay, the article that has been linked specifically talked about conviction - that means, if my English is not failing, a prosecution and a guilty verdict. Don't really know anything about Belgian law, so I can only interpolate from what law I know. Here we also don't sentence insane people, the court gives them special protective measures (one of which is indefinite internment in facility).
My first impression was that he was found responsible for the crimes, but did them because of some underlying cause. So that was wrong.
That said, it comes down to money, probably there wasn't enough allocated for department of justice to build up a interment facility for insane people, but the Belgian lawmakers were too overzealous and added such legal requirement for government to penal code. The lesson from this is, just because you legislate something, doesn't mean it is fulfillable in practice.