That's how the law of Arizona reads. If you don't like it, get it changed.
Where was the border patrol? Handing out coffee, donuts and a map of the USA?
Probably somewhere else, arresting or transporting other illegal aliens. It's not as if there are enough to have one just waiting for every illegal alien crossing the border.
You would absolutely s**t yourself if you started figuring out how many people we would need just to man the border 24/7 at just five agents per mile.
Please do NOT blame the Border Patrol agents on the ground, there are far too few of them, they are not the problem, the problem is in Washington D.C. The guys out there on the ground do a good job with what they have to work with.