The idea that the police can simply delete what they don't like or switch of the device when ever they feel like it, is a very false premise in terms of the law. Lost evidence immediately creates doubts about the veracity of statements against those who lost or purposefully failed to gather evidence. As the public and hence juries becomes more aware of the requirement of police officer to record all their policing interactions with the public they will demand that evidence, when police fail to submit it for what ever reason, it will place great suspicion on their actions and create the doubt necessary to break their prosecutions.
Not that bringing up these questions is not important, it is very important in the creation of successful secure devices that will protect the public whilst providing evidence of crimes. It will also help with the recording of crime scenes ie multiple view points. Storage of data should reflect statute of limitations for the crimes applicable, keeping in mind it does not need to be 28 frames a second, lower speed higher resolution images would be preferable.