The recent protests by immigrants and their supporters to publicize the plight of aliens trying to get to legal residency status in the US has created a lot of feedback. Many folks support immediate legal residency. Many support immediate amnesty and citizenship. The government of Mexico demands that the US allow free access for Mexican nationals who want to come for employment. I don't recall one country unilaterally demanding unrestricted access to another country except in times of war, and in the form of armed invasion.
We don't know who comes across the border. Like Sturgeons rule, probably 90% or more are upstanding folks who want work to build a better life for themselves and their family. Sovereignty, the concept of borders, demands that we maintain the capability to stop people we don't want from entering our country.
Mexico's economy depends on income earned by Mexican nationals working in the US flowing back to Mexico. 20% of people currently alive and born in Mexico now reside in the US. Does anybody think that if the US border were 100 miles north that San Diego would be the thriving metropolis that it is today? Why don't we all agree that Mexico is being poorly run, that the fine people of Mexico deserve better, and that the US should run Mexico as a territory? A large portion of the Mexican people apparently already believe that. Vicente Fox apparently already believes that. What would the downside be?