The premise establishes that there would be a major financial loss because of a fine. Whether there would be a financial loss for another reason if the fine were not imposed doesn’t change the fact that there would still be financial loss.

The argument doesn’t need to define what constitutes a major financial loss. The premise establishes there will be a major financial loss; exactly how much money is lost doesn’t affect the reasoning.

The author overlooks the possibility that the benefits of striking might outweigh the negative impact of a major financial loss. So striking might be worth doing now, despite that loss.

The author doesn’t assume that financial strength is the most important factor in the union’s bargaining position. Although the author does assume that a major financial loss is negative, we don’t know whether it has anything to do with bargaining position.

The author doesn’t assume that there will be a better opportunity to strike later. The author might believe that the union shouldn’t strike later, too.