Actually, I can presume anything I want about anyone I want. I just can't act on it. That's when the evidence has to be presented to a court (usually via law enforcement authorities, but that's not a hard and fast rule). The court can then issue warrants, if they believe the evidence constitutes sufficient justification for further investigation or arrest.
I have security cameras on the front of my house. They snap pictures of everything that passes in front of them. Should a burglary of my house or the neighbors occur, it is quite reasonable that the police/courts will base further action upon the stored video that might reasonably be related to the crime. Same idea with the iPhone.