Artificially banning a class of workers isn't really the free market. They're not leaving to make more efficient use of their labor. Just as banning black workers would up the wages for white workers according to the free market.
The ban is only 'artificial' if you believe free and open borders should be the default state of the world. This hasn't been true globally since World War I.
Surely you are not arguing that national borders are non-artificial? We can argue all day about their pros and cons, but they are certainly an artificial construct.
We're talking about in the context of economic relations, which are entirely human-created constructs. Under that definition, everything is artificial.
Fact of the matter is that every nation in the world has labor and passport controls. It would be exceptional and unusual for the US to abandon them unilaterally.
They could open the border with Mexico, just as the UK opened the "border" with Poland. If that happened, it would be undeniably a freeing of the labor market. That means the participants are individually deciding where to go, rather than other people who aren't directly involved in their trading. It might be unlikely but just because a situation is the status quo, doesn't make it natural or free.
The Soviet Union lasted a long time without a free market. Yet nobody would say allowing sellers to choose their own prices is artificial because they've been dictated by the government for decades and that's the normal and natural state of things.
