For example, do you remember the guy that caught
the video images of the Met cop killing Ian Tomlinson? Two weeks earlier the Commissioner had flatly denied any contact between the Met and Ian Tomlinson. The IPCC weren't even investigating. The cops had managed to persuade the Coroner's Office to hire a dodgy pathologist who found that he had died of a heart attack and, surprise surprise, the conclusion had leaked out into the media, even as the police themselves, who normally wouldn't be allowed to see it, were showing only a redacted copy to the family, the primary beneficiaries of such a report.
Now it turns out this passing merchant banker happened to have the whole thing on video. He went to The Guardian. By a most propitious coincidence I have yet to understand representatives of the Met Police showed up at The Guardian's offices at about the same time arguing the video should not be published. The Guardian published anyway and the rest is history.
But what if the cops had said to Mr. Banker, the copyright owner, "If you proceed with this we will of course thoroughly investigate the crime you allege against this cop, evidenced by this video, but we will also thoroughly investigate you and your hard drive, as is our job"? Could this perhaps have had an intimidating effect on Mr. Banker? Now maybe he is a rare, pure soul who can be sure he has nothing to fear and has broken no laws. Maybe he has a team of lawyers who can advise him daily on the latest legislation and can constrain the contents of his hard drive accordingly. Maybe he has never contacted anyone or visited any websites he wouldn't want his wife to know about. Then again, maybe not. Or maybe he's a poor banker and can't afford the lawyers so can't quite be sure. Yes. I think this may have an intimidating effect sufficient to swing the balance in some cases. In those cases the public at large will never see the videos and the police will be able to cover up their crimes all the more effectively.
We should recognize the legitimate authority of the police, but we should never trust them unquestioningly, and when we give them sweeping powers like this we give them too much power, precisely because we can't trust them.