Labor unions are monopolies on labor in a certain industry or factory. The workers come together and decide what the price of their labor should be, and then try to get what they want through refusing to work and keeping other people from working in the factory by physically preventing them through sit-in strikes, or picketing in lines around the factory to prevent strike-breakers from coming in to work, or they do it through intimidation or legislation. In this way, a labor union is no different than a large corporation with a monopoly on a certain good. Suppose a single corporation produced all the food in the country, and it could prevent other people from selling food by buying them up or by other means. They could then set the price of food higher than what it's worth to make a profit. This is exactly what labor unions do with labor, and it should be illegal. All monopolies should be illegal except for natural monopolies that have to be monopolies in their respective industries, and these should be regulated by the government. They restrict trade and mess with the system of supply-and-demand where everything is worth what people are willing to pay for the supply that is provided at the price that establishes an equilibrium.

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"The raw fact is that every successful example of economic development this past century--every case of a poor nation that worked its way up to a more or less decent, or at least dramatically better, standard of living--has taken place via globalization; that is, by producing for the world market rather than trying for self-sufficiency."

-- Paul Krugman