It's a controversial subject, but we live in a harsh world. Some police officers lie. Some have a documented history of misconduct, including lying or other acts that call their truthfulness into question. If they do, this history should be available to the defense so that the officer's credibility can be tested in court.
According to the National Registry of Exonerations, at least 139 people were exonerated of crimes in the U.S. last year. Some 2,100 have been exonerated since 1989, when DNA evidence was first used to exonerate someone. Today, people are being found innocent using a variety of methods that don't always involve DNA, but the principle remains the same: Sometimes, the criminal justice system convicts innocent people.