Norwegian is technically an EU airline because despite the name they're also headquartered in EU countries. This is so because being EU-based they're free to use all EU airports to base flights out of, which they use to good extent to make flights between the US and the French Caribbean islands, something that would be impossible had they been solely HQd in Norway :)

Check out this video about it (Norwegian starts at about 10 minutes in): https://www.youtube.com/watch?v=thqbjA2DC-E

edit: turns out they're registed in several countries, not just Denmark.