[yummyfajitas]

They obtain a monopoly or oligopoly through market power.

So if a union lacks market power, an employer is legally free to fire the union employees and replace them with non-union employees at market wages?

The problem being that without unions, labor becomes subject to oligopsony buying power and Ricardo's Law of Rent kicks in.

Can you explain this claim? While it's certainly true in a few narrow fields (chemistry/biotech, various specialized corners of academia), it's hardly true in the economy at large. In what fields do you believe an oligopsony is present?

[dredmorbius]

In any field in which there is a concentration of employers relative to employees. Which is to say: most of them.

Quoting from Wikipedia: "The Law of Rent states that the rent of a land site is equal to the economic advantage obtained by using the site in its most productive use, relative to the advantage obtained by using marginal (i.e., the best rent-free) land for the same purpose, given the same inputs of labor and capital."

Where "land" is taken as capital, equipment, and/or alternate business opportunities, the employee's wage-bargaining leverage, in the absence of collective bargaining, is what s/he could make by going elsewhere and starting up a new firm. Where no new business opportunities exist, wage bargaining falls to subsistence levels (employers will pay employees the bare necessities for staying alive).

In the economy at large, the situation still remains true. An employer need pay an employee no more than that employee could claim at another job (or by going into business for him or herself), given the employee's skillset.

Given that skills tend to wed to experience, should the employee transition to a different line of work (at which they are less skilled), unless there is a peculiarly high demand for that work, their wages will fall. Also, the employer's surplus (that is, productivity above wages) is governed by the Law of Rent.

Given collective bargaining, through the threat of withholding labor (with skills that, collectively, the employer would be hard-pressed to replace), a negotiation for total compensation (wages, hours, benefits) in which more of the employee's surplus is distributed to the employee may be arranged.

[yummyfajitas]

In any field in which there is a concentration of employers relative to employees. Which is to say: most of them.

This is true not just of the employment market, but of most goods markets. There are far more buyers of cars/computers/cheese than sellers. Does this mean that pizza producers have oligopoly selling power?

Your theory is too broad. It applies to virtually everything.

In the economy at large, the situation still remains true. An employer need pay an employee no more than that employee could claim at another job (or by going into business for him or herself), given the employee's skillset.

Conversely, an employee needs to accept as wages no less than the wage he could get from another employer. This is true of any market - a purchaser needs to pay no more than market price, and a buyer needs to sell for no less than market price.

All you are describing is market pricing. Are you claiming all markets are oligopolies or oligopsonies?

[dredmorbius]

The distinction between employment and sellers markets is that an employee typically has a (mostly) exclusive relationship with a single employer, and relatively high switching costs (interviews, unemployment), particularly for more advanced / skilled professions.

A buyer usually has very low switching costs between merchants. That said, yes, there is a great deal of concentration in retail, especially as you go back up the supply chain. For electronics and other advanced goods, there is typically one or a very small set of manufacturers (at least at the component, if not the finished product scale), e.g.: Foxconn for laptops and mobile devices, a handful of disk drive and memory manufacturers, etc. For food, there's a huge level of concentration at the mid-market, through Monsanto, Tyson, Cargil, Con-Agra, etc., despite the huge number of individual food outlets.

While I don't claim that all markets are oligopolies/oligopsonies, a great many are (or exhibit a great deal of concentration, or of market distortions such as healthcare) including many of those comprising a large share of the US economy: health, finance and insurance, utilities, retail. Enough so that what we consider to be the conditions of a free market: economically small buyers and sellers with equivalent information and low switching costs meeting in an open marketplace, are actually met in only a small portion of the economy as a whole.