If they step foot in our country it changes the entire dynamic. A wall or fence would prevent alot of that from happening.
But why are we allowing them to step foot in the country to begin with? And yes, I understand there is a difference between entering illegally via a gap in the fence, a tunnel, water, etc vs coming through at a checkpoint - but the question remains, why allow that? If I drive up to Canada, lose my passport while I am up there, and try to drive back across, I'll be told to pound sand. Why is there a different standard for Mexicans?