Look at it from the perspective of an investigator who believes that there are Americans providing financial assistance to a caravan that turned violent, as wel

Look at it from the perspective of an investigator who believes that there are Americans providing financial assistance to a caravan that turned violent, as well as the ordinary customs enforcement duty to ensure that large amounts of cash do not cross the border. If you don't know which Americans were involved, but suspect several who were present at a particular event, it may make perfect sense to question them every time you see them at the border, and search them (at the very least) for money. The fact that a lot of the intrusive questions that the reporter complained about were financial in nature (e.g. "Do you own or rent your home?" or "Who pays for your travel?") seem entirely consistent with this mission. I agree that it is surveillance, but you seem to forget that that's one of the things law enforcement does in an investigation.