A Note On the Pay of Undocumented Workers

A friend e-mailed the following question to me in response to my claim that the compensation received by undocumented immigrants is lower than it would be if the U.S. government placed no restrictions (other than those that it places on American citizens) on the ability of immigrants, documented or not, to find work and keep work in the U.S.:

But Don.. if they receive a depressed wage because of the fact they are "illegal"… then how does that square with economic theory of free market...