The U.S. business community relies on low-wage immigrant labor to keep costs down. Agribusiness in particular depends on migrant labor. Low-paid Mexican farm workers who remain in the U.S. illegally help ensure American crops are priced competitively in world markets. The Federal Reserve also welcomes the presence of low-cost labor in the U.S. as it helps limit inflation. Low wages prevent a wage-price spiral from emerging given the Fed's recent emphasis on quantitative easing.
There is plenty of reason to believe that keeping the U.S. southern border loosely protected serves the interests of the U.S. business and financial elite. That is why the USG tolerates the incursion of Mexican federal police across the border, no matter how brief.