Some Strawman with that Red Herring, Sir?

Bryan Caplan of Econlog approvingly references this quote from a letter by blogger Don Boudreaux to the WSJ on immigration:

[O]pponents of openness often allege that immigrants come here to free-ride on taxpayer-supplied welfare. That this allegation is a canard is revealed by the innumerable restrictions that Congress puts on immigrants' options to work. If limits on immigration were truly grounded in fears that immigrants are largely shiftless spongers, why would Congress spend so much ink and effort preventing immigrants from finding gainful employment in America?

Huh? What? WTF? No, we allege that immigrants a) undercut the low-skilled wages of American citizens and b) are a drain on our health, education, and law-enforcement resources, among other things. But I've never encountered any allegations even resembling a charge that immigrants are "shiftless spongers." Where'd they come up with that one? You almost get the feeling that these guys have never even bothered to listen to an actual anti-immigration argument. The good news is that a few of the guys on our team go to town in the comments - not that Caplan or Boudreaux are listening.