Most self-described libertarians rail against the "anti-trust" laws, which purport to prevent companies from colluding, price-fixing, cornering markets, etc. Yet when workers form an organization to offer labor en bloc at a premium price through negotiated contract (as opposed to a la carte by the single worker under "at will" conditions), all of sudden they're "anti-competitive." Hogwash.
Yes, a union work force will generally demand (and get) higher wages under contract than a single worker would be able to negotiate on his own in "at will" employment. Yes, a union work force may well demand (and get) a "closed shop" agreement under which the employer will agree to hire only workers provided by the union.
By the same token, a company with a factory at which it manufactures CPUs in large quantity and of known quality will generally be able to demand (and get) a higher price for those CPUs from a computer manufacturer than will some guy who pulls up at the front gate with a trunk full of chips and a good story. As a matter of fact, there's a very good chance that the company with the factory will be able to negotiate an exclusivity deal on the provision of CPUs for that other company's computers.
In both cases, going with the larger, more reliable provider can be a good thing for the buyer.
Yes, unions demand a higher wage -- and that wage tends to keep the worker on the job for longer. Turnover in union shops is a small fraction of that in non-union shops. This means that the company isn't constantly fronting money to train new workers who aren't yet able to produce at a level which turns a profit for the company. In some cases, unions actually pre-train workers so that they have a good grasp of the job before they show up for their first day of work.
Knapp also makes the point that the primary impediment to union-management relations is often the State:
The state has always been involved in the labor market, and always on the anti-market side.
In the 19th century, government police and troops brutally suppressed strikes and murdered striking employers so that employers with friends in government could avoid paying market labor rates.
In the 20th century, a dog's breakfast of regulation benefited unions in some areas and aided their suppression in others -- distorting the labor market in both cases.
In areas where the political establishment favored (and was supported by) organized labor, unions ran amok, bleeding companies dry with unsupportable demands for higher wages and more benefits. The police and troops who had once shot down striking workers now stood idly by, looking the other way as union muscle broke windows, set fires and beat up "scabs" to get what they wanted. The law held the employer down while the union worked him over.
In areas where the political establishment opposed organized labor, that establishment was supported by employers who loathed the idea of paying market rates and wanted unions suppressed. Since doing so by the direct route -- call out the National Guard and crank up the machine guns -- had become socially unacceptable, they turned instead to "right to work" laws which ensured that even employers who thought a union was offering them a good deal were forbidden to negotiate exclusive contracts, and which required unions to represent, protect, and negotiate on behalf of workers who decided they didn't care to pay for that representation.
The onmostly significant point I think Tom misses in his post is that the "state" in this particular sense boils down not to some faceless entity, but the sum of individual politicians trolling for votes and/or massive campaign contributions. [Of course, Tom might argue that such is pretty much the definition of the State in some senses.]
There are some other complications--like who is in charge of regulating workplace safety and how those mechanisms should be set up--but I think Tom's point is pretty clear: there is no inherent contradiction between libertarianism and labor unions. In fact, in the anarcho-capitalist stateless society that Tom favors, they would be essential.