* To accuse L&O of "liberal bias", one must have missed several years worth of episodes, particularly the mothership's season four episode "Sanctuary". As for this particular episode, "Monster" was based on two cases - the Robert Chambers case and the Central Park Jogger case. In the latter case, the five defendants *did* have their convictions vacated. It has nothing to do with "liberal bias".
*SPOILER!* * Perps are rarely convicted on physical evidence. The main reasons are 1) the perp confesses; 2) accomplice testimony (corroborated); 3) the suspect divulges information only the killer would know - cracking out of turn, as David Mamet would say. Det. Goren knew someone had pulled the nightgown down from the way it was bunched up beneath the victim's body. But there was no way for Det. Marston to know -- unless he was the killer.