The best argument in favor to open borders I have heard is the libertarian one: to hire and be hired is a individual free choice, and governments must not interfere in it. Ask Ayn Rand.
The worst argument is that you are talking about, the Fox's argument and actually the always present argument. "Americans are bad because they don't want to share their wealth with us" Oh selfishness!
There is a sort of economic blindness in we Mexicans, about the origin of wealth. United States is seen a a sort of "El Dorado" magic place, where you can collect gold from the street.
United States is wealthier than Mexico because is a technologically more advanced than Mexico. This is a statement that any Mexican would deny. "They are wealthier because they stole us. Steal them back is a virtue".
A lot of my fellow graduates of electrical engineer are not able even to solve a simple circuit. When they go to work to United States, they must work in McDonalds and not as an engineer. Why? "Because American are racists" When I ask them about taking the EIT test they are surprised to learn the existence of EIT or PE, and that having a degree is not enough in U.S.A. to be regarded as an engineer. However, after they are fully informed, they said: "oh, no, I cannot do that, it is too hard, I would need to study AGAIN". They say it as if study is something you only do when you are younger than 25, and finished the ordeal, you will never do again.
Once I talked to a teen boy who has just finished middle school in Mexico. They called him "El Mudo". I asked him "what high school are you going". He said to me "I don't need to go to High School, I have a green card". When Fox or Calderon lobby for a more open border, I always remember "El Mudo" and think that any stuff that make you feel that education is not necessary is a coarse. Could Obama a issue a "Mudo Law"? Any permanent resident who fails in getting a High School Diploma losses the Green Card.
The way to go out our problems in Mexico is and always will be education. But in Mexico, most think that education is a piece of paper. Maybe they are true because here, in practice, a fake piece of paper is indistinguishable of a real one. But if we want to change things, we need real knowledge and for this, we need to step down from the cloud we are now. And yes, stop blaming the Americans for everything what is wrong.