Because it is based on pre-stated principles, which the other party is capable of fulfilling. This makes it non-arbitrary.

I don't understand how my statement of firemen refusing to put out a fire because they didn't like the person is arbitrary. If they publicly stated they do not like the person because of their politics and will refuse to put out fires for anyone supporting that party does that make it okay to not put out a fire on their house?

Not liking someone is arbitrary and subjective

Swedish unions are not subjecting Tesla to strikes because they dislike Tesla or Musk, they are doing it to ensure they sign a collective bargain

It doesn't make it okay, but yes, it does make it non-arbitrary. But what VoxPelli wrote regarding your original statement.

And important to note as well: Swedish firefighters are not refusing to put out fires at Tesla workshops

Because it relates to a labor dispute?