That's the point: if you artificially increase the labor pool with desperate immigrants (legal and otherwise) then you've tinkered with the laws of supply and demand. If you make it possible to hire people for less money this way, you are interfering with the natural balance of labor's "worth." The author understands that quite well.
You espouse the Labor Movement's case that all jobs should be union and they will tell you how much they should be paid. Any nonunion worker is artificially increasing the labor pool and tinkering with the laws of supply and demand ...