This is a question of trust. We can’t individually check up on every police officer, so we have to trust that they will do their job honestly and thoroughly — but we also make sure that there are people whose job it is to check this, just in case. Think about doctors: do you trust doctors to pay attention, do their job properly, and act in the best interests of their patients? Of course — and almost all doctors do. But we also have the General Medical Council, which checks up on doctors and can step in if a doctor does something wrong. Similarly, we trust the police to do their jobs properly, and if anyone has concerns that the police are doing something wrong when they access people’s data they can complain to the Investigatory Powers Tribunal, whose job it is to oversee this.