From our economics professor friend Steven Horwitz:

Jonah,
Couple of thoughts:
Unions like minimum wage laws because they make it harder for non-union labor to underprice them and bid work away from them. Raise the minimum wage and you reduce the wage differential between union and non-union workers, making union workers relatively more attractive (or less unattractive anwyay). It’s hardly altruism; it’s classic special interest.
The history of minimum wage laws is fascinating as well. In the US in the 30s and 40s, they were used as a weapon of xenophobia to prevent immigrant labor from underselling white American workers. The union proponents of minimum wage laws were shockingly explicit (to our ears anyway) about their reasons for doing so during the congressional debates when the first laws were passed. Today, of course, the minimum wage does the most damage to young people of color. You can track increases in the real value of the minimum wage very closely with increases in the black-to-white unemployment ratio, especially among the young.
One thing I always like to ask my leftist minimum wage supporting friends: why was establishing a “rate-for-the-work” law (the equivalent of minimum wage laws) one of the first priorities of the white apartheid government when they rose to power in the early 20th century in South Africa? If minimum wage laws really do help the poor, who are disproportionately of color, why would racist whites move so quickly to enact them in South Africa? The answer is that the left needs to check its premises.