I wouldn't fault you for not having heard of it; most liberals aren't far enough left/don't put enough thought into the specific issue of immigration to have such a policy; and while I'd imagine that the "international" part of international socialism is where the idea comes from I'd be pretty surprised if less than 80% of US citizens holding such beliefs identified as libertarians. I simply run in different circles.
A response essay written by Eliezer Yudkowsky posted at Cato Unbound for the issue Brain, Belief, and Politics:
Is That Your True Rejection? by Eliezer Yudkowsky
The lead essay has been written by Michael Shermer:
Liberty and Science by Michael Shermer