You're right, a human driver might not have remained in place. And as a result they would have risked further injury or death.

"“When it comes to someone pinned beneath a vehicle, the most effective way to unpin them is to lift the vehicle,” Sgt. Kathryn Winters, a spokesperson for the department, said in an interview. Were a driver to move a vehicle with a person lying there, “you run the risk of causing more injury.”"[0]

[0] https://www.sfchronicle.com/sf/article/cruise-driverless-tax...