The police are not worthy of the trust placed in them

"Mr. Keane, in his Chronicle article, offered two major reasons the police lie so much. First, because they can. Police officers “know that in a swearing match between a drug defendant and a police officer, the judge always rules in favor of the officer.” At worst, the case will be dismissed, but the officer is free to continue business as usual. Second, criminal defendants are typically poor and uneducated, often belong to a racial minority, and often have a criminal record.  “Police know that no one cares about these people,” Mr. Keane explained."

Which one of the reasons that the police are against being recorded while they do their job.  They know that being held accountable for their actions will show just how rampant their violations of the law they are sworn to uphold, actually is.