Why aren't police officers who perjure themselves under oath accused of perjury?

The first OWS protester arrested in New York has been acquitted at trial. The police blatantly lied in this arrest. Yet while the accused was acquitted based on a wealth of video evidence (including the police department’s own video) showing the arresting officers were lying under oath about why they arrested him, no charges of perjury have been brought against the police.

Police perjury is a routine occurrence. It happened to me when I was in my twenties. I had to plead guilty to trespassing when it absolutely wasn’t true. My attorney told me that it was my word against three cops, and that if I didn’t accept the plea, they would go for the maximum penalty 96 months in jail and a $1,000 fine) for the misdemeanor instead of a $50 fine.

When police officers lie on the witness stand, they can put innocent people in jail or force them to pay fines for things they did not do. Why don’t these crimes get prosecuted in cases like this one, where the perjury is so patently obvious?