>>but in just societies that isn't an option.
Someone offering people work on terms that you think are inadequate is not unjust. Interfering in private interactions between other consenting adults, to deny them the option to engage in some voluntary interaction, is what's an unjust act, that is predicated on an unearned sense of moral superiority.
And such an interjection doesn't improve the opportunities available to workers in general. If it were that easy, we could set a minimum wage of $500/hr, under which all employment offers are illegal, and create instant prosperity.
The correct wage level, for maximizing economic development, and by extension, wage growth, is set by market forces, via the intersection of supply of labor, and the output that people are willing to trade for that labor.
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.
Usually what is the case is that they use these people as workers (Making it so that they can't set their own rates or contract freely like someone who isn't an employee could) but then call them something else to avoid workers protections and liabilities.
With respect to the first point, forcing companies to overpay for labor (whether directly, through above market wages, or indirectly, through reduced management flexibility, or more costly benefits) results in higher unemployment, as previously profitable hiring opportunities become unprofitable, and a less optimal allocation of company funds, that leads to less economic development, which is the source of all wage growth.
Also mandatory benefits are a cookie-cutter solution that reduce the flexibility that employers and workers have to reach terms that maximize the benefit the worker enjoys with a given expenditure of resources by the employer. For example, the mandate may require 4 weeks of paid leave a year, while a particular worker may prefer less paid leave, and instead higher hourly wages, but the mandate prevents this. Given companies have a limited amount of funds available to spend on labor expenses, there is an opportunity cost attached to every mandatory benefit.
