From what I have seen, Norwegian criminal systems are not based on punishment. They are based on rehabilitation.
This, to me, seems like the best and most sensible route for Breivik. If he is so severely mentally ill that he can't be rehabilitated, he'll be kept inside somewhere for the rest of his life, and what good would it do to anyone to punish a man who is never going to be free to murder again? If he can be rehabilitated, then he can be rehabilitated, and all well and good - with his severity of crime and craziness (I read a good portion of his manifesto), I doubt he'd be able to get over it though.