In the broad sense, Mexicans to Americans are as Afghans to Persians; that coupled with the provisions of NAFTA may give some validity to the perception that they are neighbours and have "special" status I suppose. However, I agree that the issue of immigration should be treated equally across the board.
Politicians, unfortunately have reactive approach to most problems as opposed to being pro-active. Therefore, they sometimes try to score some points with other countries and parties and so on, and in order to achieve political longevity, they often do the "popular" thing, as opposed to the "right" thing which will offer lasting solutions.
Immigration in general is a broader global issue and has to do with uneven distribution of wealth. This is why people from poor countries try to immigrate to wealthier countries in order to secure better financial fortunes for themselves and particularly for their children. Admittedly we have no choice in the place of our birth which is also tied with certain fortunes or misfortunes. Since there certainly is no justification for a superior race, nationality, religion, gender, or people, therefore it is "unjust" for anyone who is born into one of these misfortunate regions to be relegated to third world class citizenship and abolished to a lower form of life.
The solutions proposed so far have all been treating the symptoms of this unequal distribution of wealth as opposed to eradicating the root cause, otherwise inhabitants of this global village should be free to choose where they live and a make a contribution to human civilization.