Didn't used to be. But it is now. As everyone in the universe must know, the jury refused to convict Casey Anthony on the charges of murder and child abuse. They did convict her on charges of lying to the cops. In short, the prosecution lacked enough evidence to convict on the serious charges, but after three years of investigation and interrogation they found some inconsistencies in her answers and heh presto, we got something we can pin on her.
If, God forbid, the prosecutors came after me and grilled me for three years, and kept notes, they would be able to catch me on something I said. As a defendant, your best bet is never to say anything to the cops. With or without your lawyer present.
If the prosecutors cannot get the jury to convict on serious charges, the defendant ought to go free.