Thank the honey biscuits

they're making sure it's not carrying concealed weapons

They're keeping the child in the safest place at the moment. The detachable car seat carriers are heavy with any sized child after extended lengths, and by leaving the carrier on the ground it lets the child not get jostled as they have to move and fill out paperwork. They're probably waiting on a patrol car with a carseat/matching base (the part the child is in needs the base to secure into a car).

It's almost like, in some strange way, the carjacker did a public service.