MM, I think you are right. I read yesterday that the reason that the suspect in the JonBenet case was not handcuffed in the Thailand airport or on the plane was so the investigator that was escorting him could say that he was not in his custody. Therefore anything he said could be used. They were allowing him to drink several alcoholic drinks during the flight. I guess they were hoping his tongue would get real loose.
It's NOT against the law. Courts have allowed Police to lie to people. Sometimes guilty people confess when they feel trapped by lies, witnesses, and/or evidence that doesn't exist.
The problem is that many people are weak and they've never experienced anything like a police interrogation - and sometimes these tactics result in FALSE confessions.
They had a case where they told a man that his Mother had implicated him - there was no other evidence in the case, in fact, it looked like someone else did it. The man in despair that his Mother believed that - gave up - signed a confession. I believe he was freed years later when other evidence came to light. But many in the public were shocked to find out Police and DA's can legally lie to suspects.
It's legal - and the oldest trick in the book.