There may still be other horses in the stable that would run off, my friend.
Sure, and if you want the laws to be made harsher in anticipation of their deeds, go right ahead. I'm talking about the one of Ut°ya infamy.
Either way, as was just said, the Norwegian justice system aren't about punishment or retaliation, but about rehabilitation and the safe-keeping of dangerous individuals (such as murderers, rapists, and, of course, terrorists) so that their victims don't have to live in fear of running into them on the street. We're not like certain other countries that subject people to long sentences in deplorable conditions or kill them outright to make other people feel good, the victims will have to get their therapy elsewhere.
In the case of Breivik, though, it's far from a given that he'll be let out on the streets in 21 years. He's a very special case to say the least, and it's a bit of a "wait and see" situation right now. Far from unlikely that he'll be in for life, though.