Libertarian views assume that free markets regulate themselves basically due to perfect competition. (Trying to keep this short and simple.) Realistically, you

Libertarian views assume that free markets regulate themselves basically due to perfect competition. (Trying to keep this short and simple.) Realistically, you do not always have perfect competition and you have problems with monopoly, oligopoly, etc. Firms are potent enough that their decisions can manipulate markets, and in this it would be labor markets. (Perfect competition assumes that no single firm can manipulate markets.) Libertarians oppose a government presence in markets because they believe the result is inefficiently functioning markets, and we are all worse off. In this case, the firms are manipulating labor markets so that they behave inefficiently to the advantage of the manipulative firms and disadvantage of labor. (This is usually where people try to justify labor unions as a counter to the influence of firms.)