Breivik was the man behind the most successful individual act of terror, in fact it was only pure luck that his bomb in Oslo didn't kill more people (an underground parking basement lead some of the force of the blast away). If he had been treated as American justice systyem would have treated him, he would have become a grand symbol of ... well what you wanted him to be symbol of, for the extreme right he would have become a freedom fighter for us other he would have become a symbol of evil.
The Norwegian prison system on the other have succeed in making him a giant joke, can any people imagine people seeing him as an inspiration anymore?
For a megalomaniac narcissist (like most terrorists) like Breivik that hurt more. Here was a man who planned to be killed (before he chickened out), becoming the centre poinjt of an uprising against the state, now he's seen as a petulant manchild.
I seriously doubt he would have garnered major sympathy regardless of how he was sentenced. Also, so what if his "image" is hurt more by this? I don't want to hurt his image, I want to hurt HIM.
