In many places, there are laws against collusion by market participants, e.g., doctors can't talk with each other about their prices, the CEOs of GM and Toyota can't coordinate on whether to have a sale, and the CEOs of Apple and Google can't agree not to hire each other's employees (heh). These laws often impede the formations of unions, which are collective groups of employees who coordinate on wages, benefits, etc. By repealing anti-competitive laws, or paring them back so they do not apply to unions, this can allow unions to form where they would not. The argument is that allowing monopolies (or at least oligopolies) in the supply of labor is a way to balance the negotiating advantage that businesses have on account of natural monopolies/oligopolies in the demand for labor.