As someone who's been a victim in a situation like this, I'm totally on the side of the cops. We need to draw lines somewhere. There's no good argument for protecting ownership of the kind of subject matter that was deleted here.
Edit: I would encourage anyone who's downvoting me to worry less about their own digital rights and spend some time thinking about what it would be like to have explicit pictures of yourself in the hands of someone who hurt you.
I see no indication that anybody is arguing that they should not have deleted the images in question, and the police argument appears to have been more about making sure that all contact information across various apps was deleted anyway. The arguments are a) does that mean all the data on the phone needed to be deleted? (Doubtful) b) does that mean that none of the data on the phone could reasonably be extracted first? (doubtful)