A Canadian lawyer on CBC radio, As It Happens, last night said that visitors to the USA have very few, if any, rights at the border or airports and can be denied entry for the slightest of reasons.  The latest requirement is for the border officer to ask for your cell or smart phone and passwords.  If you decline, you are denied entry and if you comply then the content could deny entry.  Border authorities have very complete power to deny entry if they see fit.  I understand that Homeland Security issued these orders recently.