Join Date: Dec 2004
Location: Riding the rails
Programs: Japan Forum, Skyteam Elite Plus, BW Diamond Select, HHonors Gold, NWA, DL, NH
Posts: 1,936
I don't know the law, but CBP does have very broad authority in searches that are beyond the realm of constitutionally protected rights and questioning, though they can't deny a US National from entering the country.
But flying in and out a bit for the last few years, CBP officers have never asked the purpose of my visit back into the US, just the purpose of my visit outside the US.