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.
There's nothing shocking about it. Our constitution and our system affords suspects and defendants many rights that most legal systems around the world do not. I have no problem with interrogators lying to suspects.
I don't think we should use this case as an indictment of the entire judicial system or the investigative process because this case is an anomaly to be sure.