An excellent follow-up article by Paul Karl about his remaining silent when interrogated by CBP agents at an airport:
This is principally about the right to silence. CBP officers are law enforcement (pictured), who can detain you, arrest you and testify against you in criminal court. You place yourself in jeopardy every time you speak to them about anything.
CBP officers are not your friends. CBP officers treat returning U.S. citizens as potential criminal defendants. You should likewise treat them as if they were corrupt cops on a power trip, targeting you to goose their arrest statistics. The best way to protect yourself against their depredations is to refuse to speak to them or to answer their questions.