It's not a gift from Norway, it's a gift from the City of Oslo.
And the reasons are far more complex than the official 'thank you' that is quoted.
After WW2, Norway was a very divided country. Although the King and Government had moved to the UK, huge numbers (15,000+) had volunteered to join the Germans including over 6000 that had clamoured to get in the SS. Some were in mixed German/Norwegian units but there were also whole SS divisions recruited solely from Norwegian volunteers that fought with enthusiasm and distinction in all theatres of the war. There was no German occupied country that provided such a high percentage of volunteers to the SS.
The post war years were a time of retribution and revenge by the allied aligned Norwegians against the returning ex-German army and SS volunteers and their families. The 'resolution' was brutal with many deaths in the internment and prison camps. A large number who fought against the Russians were given over to the USSR to die in their death camps.
The Norwegian Government was extremely keen to send every signal possible that they wanted to continue to be aligned with the west. Part of their 'adjustment' to the image of their collective war service was an accentuation of the links there had been with the UK (with the gift of a tree, ski schools in Scotland, cultural exchanges, etc), and then as the cold war dawned, their enthusiasm was fervently embraced as they were key to the defence of NATO's 'northern flank'. It was seen by everyone that the story of their war service should be represented by 'The Heroes of Telemark' and not the alternative, but far more accurate German offering, 'Viking Division does good job trampling on European Babies'.