I'll grant that so long as the immigration is illegal there's a case to be made for enforcement of existing laws.

But what I don't quite get are people like Tom who are generally libertarian except when it comes to immigration. Why should the government block entry into the labor market any more than it should restrict other private economic decisions? Clearly, global welfare and global liberty would be increased if labor as well as capital could flow freely, so why should we have strict immigration controls to protect the current, distorted allocation of labor rents? Libertarians don't make that argument when it comes to protecting other existing but no-longer efficient allocations like protecting U.S. auto manufacturers or farm subsidies to protect "the rural way of life".

To be fair, most libertarians _don't_ make that argument -- most favor opening up immigration. But there's a significant minority of self-styled libertarians who rail against government intervention but who also want to close the gates and limit entry to the U.S. labor market. I don't get how you make that moral/intellectual case other than, "what's good for me".