I don’t know for sure, but I think it is more likely that it is his choice.
Norwegian royal expert Carl-Erik Grimstad
stated in 2009 that no Norwegian authority has ever ruled on whether a royal title is part of a name.
Moreover, Norwegian royal expert Dag T. Hoelseth
has pointed out that even though
the Name Law requires everyone to have a surname, and there is no exception in the law for titled people, the authorities nevertheless allow the royals to not register any legal surname.
So, even when the legal requirements are clear (I don’t know whether there is a legal requirement to use one’s legal name in a business registration), the authorities are willing to allow royals to technically break the law.