Cars can be hacked as it sits.
If it has automatic parking assist, cruise control, and computer aided traction control that uses the brakes, your car can be subverted.
Now, imagine that with a robocar where you are out of the loop.
Just because it has a computer does not mean it can be remotely controlled. Cars have had dozens of computers in them for decades now, yet you never hear of them being hacked.
My expertise is in computer chip design (hence the username), and people often don't understand that not everything has remote wireless access, and that it is easy to segregate different systems in the car that may have remote access.