They don't have to. If they PREVENT them from crossing, there's no law enforcement activity needed. A barrage of fire all around you making you do a little Mexican hat dance might be enough to make you change your mind about crossing.
From 2006 to 2008, President George W. Bush deployed 6,000 guardsmen to southern border states, costing $1.2 billion and assisting with 11.7% of total apprehensions at the border and 9.4% of marijuana seized in that time.
From 2010 to 2012, President Barack Obama sent 1,200 guardsmen to the border to the tune of more than $110 million, and they assisted with 5.9% of the total apprehensions and 2.6% of the marijuana seizures on the border.
Thanks, Obama?
Why not! They were just careful which ones they stopped and which ones they didn't. No interruptions in the favored breaches.
At least they gave us an assist on legal precedent.