The Public Interest Litigation Project (PILP) of the Dutch Section of the International Commission of Jurists (NJCM) has conducted an analysis into the role and responsibility of airline KLM, who denied passengers the right to board. The denial was in compliance with the immigration ban issued by the President of the United States of America... Based on an analysis of the order, it appears that the immigration ban did not qualify as a reasonable ground upon which KLM could refuse passengers from boarding... There is no prohibition under either American or Dutch law from carrying passengers on flights in such instances. There is, however, a prohibition against refusing them under international law. To do so amounts to discrimination on the basis of prohibited grounds, in this case nationality. In carrying out the discriminatory policy of another actor, the USA, in the form of the immigration ban, KLM could be considered as contributing to human rights violations...

You can find the full analysis in Dutch here.