There is a subtlety here that helps defendants with competent lawyers. The police and prosecutors will be more than happy to show video evidence in court when it justifies police actions -- it is pretty hard to raise doubts about the claim that a suspect charged at the police with a knife when the police have a video of it happening. When the police show up in court and claim to have forgotten to turn their cameras on, the defense gets a big boost -- "Convenient that you forgot to turn the camera on just before beating my client into a coma, yet you did not fail to turn it on in your previous 30 arrests."