An abuser would know if the device was failing to report a location, or if the victim were to turn off the system, and could take action accordingly.

When user consent can be subverted, otherwise benign opt-in features that are deployed and marketed ubiquitously can have unforeseen consequences.

That said there is a utilitarian way to look at this, that for more situations it helps parents to keep their kids safe. The ethical question is far from cut and dry.