Norges Bank informed me that as Mint of Norway is since 2003 no longer owned by the Norwegian government and as tenders for the production of the Norwegian coins are regularly being held (and so it is possible that other companies may in the future produce the Norwegian coins), what I gather from their message is that they felt it better and more 'official' to not allow these private companies to add their marks but instead use the NB mark of Norges Bank.
I can see what they mean but on the other hand, these governments should understand that the adding of mintmarks on the coins is a tradition that goes back a long time and I don't think it would be good to scrap this tradition just because mints are no longer state-owned and the mintmark may change from time to time when another mint wins the tender.
I find that the NB mark for Norges Bank is just meaningless on the coin.