You want it both ways
You want surveillance when it may be to your advantage
You do not want surveillance when it may work against you
You're missing the entire point of cameras on the police. If the police are there, you're already being watched - by the officer, if nothing else. In the absence of a camera, what the officer "sees" is whatever he says he sees.
The camera is not there so much to watch you, as it is to watch the officer. In any court, if you and an officer disagree on a series of events, the officer is always assumed to be telling the truth, not you. The purpose of a camera on every cop is to know what actually happened - the point is to put a control structure back in place so the police cannot make ridiculous claims, or plant evidence, or whatever other dirty tricks they can use when unsupervised. Not all cops do this, but enough do that it has undermined the public's trust in the police in general. The point is as much to document police corruption and abuse as it is to have evidence for use at trials.