If a company is legally required to use PostNord and PostNord can legally refuse to deliver mail, it effectively gives PostNord the power to control any company that wants to continue receiving mail. Regardless of whether they are using that power for good or evil here, I don't think it's a power they should have.
Why not? They're a private company. Isn't that usually what Americans are telling people, that corporations are free to serve who they please?
But, more importantly, PostNord isn't doing anything - it's a unionised group of workers refusing to deliver Tesla products. That's how strikes work, and Musk's go-to course of action is to immediately proceed to legal action rather than negotiating and meeting the union's requirements.
