It prevents other workers from underbidding them, by making it illegal for the employer to hire those workers due to the contract liberty suppressing mandate to engage exclusively in collective bargaining with the union.
Forcing companies to pay more than market wages is also bad for society at large by discouraging investment into such companies. More investment into companies leads to lower consumer prices which translates to an effective wage hike for all workers.
And as it would happen, giving a select group of 'little people' special privileges, through coercive force against employers, results in higher consumers, which hurts all other little people and/or less investment, which reduces future wage growth, for all workers.
Reread my comment, they were never able to underbid each other in the first place they already had a fixed hourly rate, the strike and subsequent contract negotiated a higher rate so nothing to do with their ability to underbid was changed since it was never possible. So I'll ask again, how are did this specific incident restrict the rights of other workers?