Many folks on Twitter, Facebook, GooglePlus and various online forums in the English-speaking world are outraged that, as @PiersMorgan (who has a million followers) tweeted, "this inhuman psychotic killer can only be jailed for a maximum of 21 years" . Although it's a great soundbite that is guaranteed to generate teeth gnashing (and retweets, 100+ for @PiersMorgan), it's wrong.
First, Norwegian law does set a maximum of 21 years in prison for "planned" (what we might call "premeditated") murder.
Whether that is on a case-by-case basis (21 years per person killed) and would be additive in this case is not clear. Understand the social context: the homicide rate in Norway in 2000 was 3 per 100,000 people, and the country has fewer than 5 million citizens. That's less than 150 murders a year. Crime in general and murder in particular is not the problem in Norway that it is in the United States, either in terms of rate (in 2000, the U.S. homicide rate ranged between 5 and 15, depending upon the age of the victim) or scale (15,586 murders), and law tends to reflect culture.
Second, regardless of whether or not the terms are additive, Norwegian law recognizes the exceptional event and provides for longer sentencing -- it just does so differently than in the U.S. From the Norwegian edition of Wikipedia via Google Translate (with sources, if you read Norwegian):
Containment [or custody] is one of the penalties in the Norwegian legal system. Preventive detention can lead to imprisonment for life.... [containment is] "implementation of preventive detention to protect society's need for security against the new serious crime by the detained person" ...
From a 2005 article in Kriminalomsorgen (Google Translate, emphasis added and grammar corrected):
[Containment is one of three sanctions implemented with an amendment passed 1 January 2002.] Never before [has] social protection
havebeen expressed so clearly as in the provisions on special sanction. Custody can only be used when a timed penalty is not considered sufficient to protect society. The social protection [is used] to justify the reaction, not justice. [...]
Custody can only be imposed if the person has committed or attempted to commit a crime that has violated someone else's life, health or freedom - our society's fundamental right goods. [...]
Preventive detention is ... for an unspecified period... When the time limit [21 years maximum] expires, the offender [is] reassessed. If the court concludes that the recurrence risk is still present, the time frame [can] be extended by up to five years. There is no upper limit to how many times the court may extend the time frame. In principle, therefore, a person sentenced [to] custody [is] in prison for life.
The last execution took place in 1876 when murderer Kristoffer Nilsen Grindalen was publicly beheaded on a scaffold built--according to custom--near the scene of the crime.
Capital punishment was retained for certain military crimes in wartime. During Nazi occupation from 1940 to 1945, Norway's government-in-exile reintroduced the death penalty for certain civilian crimes as well, notably political high treason.
The country's political leadership has also aligned itself with death penalty opposition throughout the EU.
I am not a lawyer, but the current law seems pretty darn straightforward to me.
Just because another country doesn't have bloodlust like many do in the United States does not mean that the country has no means of ensuring appropriate social protection. Notice, please, that I did not say justice or vengeance: the law seems pretty clear to me, social protection is the goal, not "justice" or vengeance.
This latest meme is an opportunity for me to leave you with my number one soapbox point: if something seems too good or too outrageous to be true, that's probably the case. Don't retweet, forward, share, +1 or otherwise contribute to such memes until you've checked them out.
 To his credit, Morgan tweeted a link to the Norwegian Wikipedia entry on containment. However, I think the language he used in tweet is cryptic, and he didn't tweet a correction or delete the massively retweeted tweet.