[wbg34]

There was an incidence of violence at the border. The people in this database were present at the time.

"CBP told our colleagues at NBC News that the names in the database are all people who were present during violence that broke out at the border in November. The agency also said journalists are being tracked so that the agency can learn more about what started that violence."

[thinkcontext]

If they want information relevant to a crime they can ask for an interview, just like law enforcement in the interior. If they want to compel testimony they can get a subpoena or if they want to search a device they can get a warrant. The fact that they are at the border allows them to circumvent these usual checks on law enforcement.