Thanks to Brian Doherty for his link to my work in his recent essay on immigration. But let me clarify one point with the following, drawn from footnote 23 of my Natural Order, the State, and the Immigration Problem.
It is incorrect to infer from the fact that an immigrant has found someone willing to employ him that his presence on a given territory must henceforth be considered "invited." Strictly speaking, this conclusion is true only if the employer also assumes the full costs associated with the importation of his immigrant-employee. This is the case under the much-maligned arrangement of a "factory town" owned and operated by a proprietor. Here, the full cost of employment, the cost of housing, healthcare, and all other amenities associated with the immigrant’s presence, is paid for by the proprietor. No one else’s property is involved in the immigrant-worker settlement. Less perfectly (and increasingly less so), this full-cost-principle of immigration is realized in Swiss immigration policy. In Switzerland immigration matters are decided on the local rather than federal government level, by the local owner-resident community in which the immigrant wants to reside. These owners are interested that the immigrant’s presence in their community increase rather than decrease their property values. In places as attractive as Switzerland, this typically means that the immigrant (or his employer) is expected to buy his way into a community, which often requires multimillion-dollar donations.
Unfortunately, welfare states are not operated like factory towns or even Swiss communities. Under welfare-statist condition the immigrant employer must pay only a small fraction of the full costs associated with the immigrant’s presence. He is permitted to socialize (externalize) a substantial part of such costs onto other property owners. Equipped with a work permit, the immigrant is allowed to make free use of every public facility: roads, parks, hospitals, schools, and no landlord, businessman, or private association is permitted to discriminate against him as regards housing, employment, accommodation, and association. That is, the immigrant comes invited with a substantial fringe benefits package paid for not (or only partially) by the immigrant employer (who allegedly has extended the invitation), but by other domestic proprietors as taxpayers who had no say in the invitation whatsoever. This is not an "invitation," as commonly understood. This is an imposition. It is like inviting immigrant workers to renovate one’s own house while feeding them from other people’s refrigerators. Consequently, because the cost of importing immigrant workers is lowered, more employer-sponsored immigrants will arrive than otherwise. Moreover, the character of the immigrant changes, too. While Swiss communities choose well-heeled, highly value-productive immigrants, whose presence enhances communal property values all-around, employers under democratic welfare State conditions are permitted by state law to externalize their employment costs on others and tend to import increasingly cheap, low-skilled and low value-productive immigrants, regardless of their effect on all-around communal property values.
Theoretically bankrupt, the left-libertarian open border stance can be understood only psychologically. One source can be found in the Randian upbringing of many left-libertarians. Big businessmen-entrepreneurs are portrayed as "heroes" and, according to Ayn Rand in one of her more ridiculous statements, are viewed as the welfare state’s "most severely persecuted minority." In this view (and untainted by any historical knowledge or experience), what can possibly be wrong with a businessman hiring an immigrant worker? In fact, as every historian knows, big businessmen are among the worst sinners against private property rights and the law of the market. Among other things, in an unholy alliance with the central State they have acquired the privilege of importing immigrant workers at other people’s expense (just as they have acquired the privilege of exporting capital to other countries and being bailed out by taxpayers and the military when such investments turn sour).
A second motive for the open border enthusiasm among contemporary left-libertarians is their egalitarianism. They were initially drawn to libertarianism as juveniles because of its "antiauthoritarianism" (trust no authority) and seeming "tolerance," in particular toward "alternative" — non-bourgeois lifestyles. As adults, they have been arrested in this phase of mental development. They express special "sensitivity" in every manner of discrimination and are not inhibited in using the power of the central state to impose nondiscrimination or "civil rights" statutes on society. Consequently, by prohibiting other property owners from discrimination as they see fit, they are allowed to live at others’ expense. They can indulge in their "alternative" lifestyle without having to pay the normal price for such conduct, i.e., discrimination and exclusion. To legitimize this course of action, they insist that one lifestyle is as good and acceptable as another. This leads first to multiculturalism, then to cultural relativism, and finally to "open borders." See further on this Democracy: The God That Failed, esp. chap. 10.