Bill Gertz has reported that al Qaeda members are working with Mexican organized crime groups. Imelda Ortiz Abdala, formerly the Mexican consul in Lebanon, was arrested and charged with providing passports illegally. Then there is the recent discovery of a Tijuana smuggling ring that specialized in Arabs.
Attacking areas heavy with Mexican citizens would likely reduce the flow of "migrants" making it easier to spot OTMs, Other than Mexicans. Besides many Mexicans have close relatives ILLEGALLY here and would likely take sides in the war on terror if those "migrants" were threatened.
According to one site that shows the transatlantic routes for Air France, Paris to LA, it comes down from Canada and over Las Vegas. I don't know the timing but someone mentioned a bowl game in Las Vegas. Maybe that. Not likely to be many Mexicans there.