Ace looks at the highly effective wall the Israelis put up and asks: "Are the Amnestias against the wall because it won't be effective — or against it because it will be effective?"
Opponents of the wall genuinely think that sealing the border is impossible—at least those in the mainstream do. Furthermore, if they refuse to even entertain the notion that sealing the border is possible a) they will never be proven wrong; and b) their adversaries will never be proven right. And it doesn't hurt that their stance will make them the favored choice at the polls for the very vocal hardcore believers who think that any attempt to close the borders is a betrayal of their ideals.
The woman who wrote the post compares border control to the former impossibility of controlling crime in New York. Lots of New York liberals seemed to think that crime was legitimate response to poverty and racism.