That is (unfortunately) not entirely true. If they have reason to suspect there is an immediate risk of injury or death, they can locate the phone (assuming it's on) with a high degree of accuracy (at the very least, cell tower triangulation, if not GPS activation).

We had a situation a few years ago where a couple kids had skipped school and gone joyriding in a quarry. Their parents contacted the police, who contacted the cell phone company, and the cell company gave them a reasonably accurate fix (I assume this was voluntary on the part of the phone company, given how quickly it happened).