Simply put, Jacoby, Medved and some other open border columnists are Jewish...they remember when immigration here was restricted and adversely impacted them, So they tend to be empathetic with those who want to come here, even ILLEGALLY.
But whereas Jews tend to be bright and educated, and thus become net contributors when they emigrate, MOST ILLEGALS are poor and often ignorant Hispanics. That is a formula for socio-economic DISASTER as detailed in this excellent study by an expert on immigration at the FED:
excerpt (released in 2006)--
Immigrant Assimilation: Why Worry?
Worrying about immigrant assimilation boils down to worrying about Mexican immigration. Non-Hispanic immigrants consistently outperform natives, even after three generations here.
Meanwhile, Mexican immigrants make up the largest and least educated immigrant group. While they make the biggest gains (coming from such a mean base) after coming to the United States, they STILL lag behind in education and wage outcomes even after 3 generations here.