Unions: An economic evil

Consequently, at best, a union can achieve a higher, restric­tionist wage rate for its members only at the expense of lower­ing the wage rates of all other workers in the economy. Produc­tion efforts in the economy are also distorted. But, in addition, the wider the scope of union activity and restrictionism in the economy, the more difficult it will be for workers to shift their locations and occupations to find nonunionized havens in which to work. And more and more the tendency will be for the dis­placed workers to remain permanently or quasi-permanently un­employed, eager to work but unable to find nonrestricted op­portunities for employment. The greater the scope of unionism, the more a permanent mass of unemployment will tend to develop.

... says Rothbard in Man, Economy and State (chapter 10). Now you know.