“Just as businesses seek to have government impose tariffs on imported goods that compete with their own products, so labor unions use minimum wage laws as tariffs to force up the price of non-union labor that competes with their members for jobs.”
There are two primary reasons unions agitate for minimum wages when union members - professionals in a given trade and not entry-level laborers - don’t receive the minimum wage. Both are, naturally, self-serving.
The first: certain labor contracts have automatic wages increases when the minimum wage increases. So when the minimum wage goes up, their pay goes up.
Second and most common (and what Sowell discusses above): it makes the union laborer more attractive to a potential employer relative to a minimum wage laborer.
For the sake of argument, let’s take it as a given that a union worker is more productive - due to training and experience and so forth - than a minimum wage worker. But how much more productive he may be is relative to the price of his labor. If a union worker is twice as productive as a minimum wage worker but he’s three times as expensive per hour, then the union worker will not be hired. If you can get the same utility from two minimum wage workers that you get with one union worker, but it’s cheaper, ceteris parabus, to hire the two minimum wage workers, then that’s what any sensible business person would do. But if the minimum wage increases, that gap narrows - potentially eliminating the discrepancy altogether. So even if the minimum wage doesn’t increase a given worker’s cost to his employer beyond the productivity he imparts to said employer, the minimum wage worker may still lose his opportunity for employment if his value drops relative to the union worker simply because he must not charge an employer less than the minimum.