Once the illegal's cross the border they fall under the immigration laws when they surrender to the Border Patrol or caught by the Border Patrol. If the Border Patrol can not stop their entry while exactly at the border why should the Border Patrol even be at the border to take and process them under the immigration laws.

Just let them cross and once beyond the Border Patrols catch zone they are in the country illegally falling under different laws. They have not been processed by the Border Patrol to be in the country legally.