They don't care about you. They do care about the local crime rate, and the amount of work they have to do when you report your phone as stolen. Don't think of this as them treating you as a kid, think of it as them saving themselves some work and money.
Not every unlocked car containing valuables is a guaranteed theft report, so rummaging through every unlocked car with visible valuables sounds to me like potentially more work and money.
Note that I'm not just interpreting the situation this way to make the police look bad. This is how they actually, explicitly describe it!
> Sharp said this plan does two things.
> “The bad guy is not going to break into the car and be able to take that item. It inconveniences the person to come down and pick up the property,” the lieutenant said.
> That inconvenience is a kind of tough love approach that reinforces the safety message.
> “Maybe next time they won’t leave their stuff in plain view,” Sharp said.