Quoting another comment on this topic I made while inebriated to back up the idea that this is economics 101 material:
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 collective decisions can manipulate markets, and in this it would be labor markets. (Perfect competition assumes that firms cannot 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 collude and 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.)
Unless those participants had absolutely no idea what they were doing, salaries did at least not rise as much as was warranted.
This is not some far fetched theory this is economics 101 applied to the real world. Asking for evidence for this claim is the real embarrasment here.