Agency pilots and cabin crew have no labor rights nor labor principles with their real employer, Norwegian - para 2 above (e.g. union representation and collective agreement).
If the Spanish agency pilots were to strike against Norwegian they could be summarily terminated without recourse. In a "direct employment relationship" striking pilots cannot be terminated without due process (recall the 600 directly employed pilots on strike in March). In the UK the difference between the Employment Act and the Agency Workers Regulations - right to strike vs immediate dismissal.
Kjos does not mind if agency pilots join one or ten unions, he has no obligation to communicate or even acknowledge any agency union, as evidenced by SEPLA's failed attempts.
The Kjos labor scheme circumvents the prerequisites of Article 17. Additionally, Kjos has stated in his application to the US DoT that agency crew are offered the same degree of job protection, terms and conditions as crew hired directly by Norwegian - they are not.