260-mile is not a huge area?
It is huge in terms of land mass, but that is absolutely irrelevant when talking about the purpose of the checkpoints.
It is between 40 and 50 miles from the border to the main transportation artery, Interstate 10. Nobody walks that, the slave-traders use vans and pickup trucks to transport their loads north.
There are only three roads in the County that take you to I-10, and each has a checkpoint. And you can't just go around the checkpoint, because there aren't any roads that go around...not even jeep trails...that the Border Patrol can't control.
Think about getting food into your stomach...it goes from mouth to throat to esophagus to stomach. Close the throat, and you're not going to eat. It's the same here...the checkpoints effectively close off traffic north.