There are limits to the lies they can tell, IIRC. They are limited to lies which would not harm an innocent person. So if an officer says "We have your fingerprints on the murder weapon", that is fine, because an innocent person will know that there is no way that can be true. Whereas a guilty person would presumably find that lie plausible.

But if an officer gets a confession by saying, "They just passed an amendment, and you don't have the right to a lawyer anymore, so you have to answer my question or I'll send you to a torture dungeon", that is a lie that can harm anyone, regardless of guilt.