Up until the 1930s there was freedom of contract between workers and employers by which they could make, accept, or reject any offers of remuneration. With the 1930s comes the idea of exclusive bargaining agents decided upon by a majority of workers, and compulsory to all.
It becomes hard to impose injunctions to stop union violence. Massive strikes, dominated by complete strangers, are permitted in this legal order. Morgan Reynolds listed seven distinct ways in which unions imposed costs upon the economy.
In an unhampered market workers will tend to do better over time. That is because of business investment in capital goods. This increases the productivity of labor. Lower prices and higher quality result in an increasing standard of living without any coercion or government intervention.
Lecture 8 of 10 from Thomas Woods' The Truth About American History: An Austro-Jeffersonian Perspective.