Just to be clear, the Border Patrol does not check anyone while they come through the ports of entry. Customs agents do that. BP agents are arresting people that skirt around the fence, run, hide, and jump into vehicles that are driven by US citizens, legal immigrants, and other illegal aliens. Those smugglers use the roads that other citizens use everyday. The checkpoints are there as a layered approach to help control an area. When you come up to a checkpoint, each agent should be wondering if you are legally in the country and whether you are potentially smuggling. Simple questions as you have described are not easy to answer when someone is nervous and trying to be evasive, such as when someone is smuggling. I doubt there are orders to harass as you suggest.
For the record, a checkpoint stop and a vehicle stop are not the same thing in the eyes of the law. Different levels of suspicion are needed and your initial stop at the checkpoint was not random as everyone entered the checkpoint. Contrary to your assertion, a policeman, during a lawful traffic stop, can engage you and ask you
questions.
If you have read the court rulings that have given the checkpoints their authority, then surely you would see how the Supreme Court took a lot of care in trying to balance everyone’s Constitutional Rights, public safety, and the need to secure the border.
I can understand that checkpoints aren’t considered ideal, but I would love to hear your solution on how to catch those entering illegally.