Striking is a constitutionally protected right in Sweden. The belief of PostNord is that the right of their employees to strike weighs higher than PostNord's obligation to deliver mail.
"Stridsåtgärder på arbetsmarknaden [K2]14 § En förening av arbetstagare samt arbetsgivare och en förening av arbetsgivare har rätt att vidta stridsåtgärder på arbetsmarknaden, om inte annat följer av lag eller avtal. "
It is. Postnord even sent a press release stating the same:
> "PostNord's position in a conflict that we are basically outside of, has been that the right to strike is constitutionally protected and therefore applies over the distribution obligation in the Postal Act," the company writes in a press release. [1]
1: https://www.dn.se/ekonomi/postnord-slar-tillbaka-mot-tesla-i...