How many (if any) monarchs, who reign over countries where it is not the cultural norm for parents to arrange or veto the marriages of their children, have chosen to publicly condemn their adult child or family member's choice of partner, especially if it would expose them to accusations of racism?
As for accepting Durek Verrett into the Royal Family: Had this been the first marriage of a princess not directly in line to the throne, it might be feasible to declare that he would not become a member, as it would be a novel solution to an unprecedented situation. However, after declaring Erling Lorentzen, Johan Martin Ferner, and Ari Behn to be members of the Royal Family (especially Ari Behn, who was also mired in public criticism and allegations), and accepting their attendance at some public events, it would be evident discrimination to treat Durek Verrett, who is in exactly the same position, differently.
As Heavs explained, it is also much more diplomatic to blame "cultural differences" than to blame an individual, or even multiple individuals. The royal families of Denmark, Sweden, and the UK have also taken advantage of similar explanations in recent years.
On a different note: It just occurred to me that in spite of the plethora of very specific details in the recent press release concerning the couple's future status and roles, at no point was there a statement such as "As the King has granted his consent to the marriage pursuant to Article 36 of the Constitution, Princess Märtha Louise will remain entitled to succeed to the throne." While we can probably infer the King's consent and it seems no formal decree is required, it is interesting that the King has so far chosen not to issue any "clarifications" on that issue.