Re: Why is the onboard computer able to control the brakes and steering?
Lots of new cars have "Park Assist", you stop beside a space, put the car in reverse, and press a button. In theory it parks in the space, which requires steering and brake control. Since the hackers noted that they could only control the car when it was in reverse I guess this is the system they used.
I test drove a few new cars with park assist recently. It's impressive in a way, but unnerving, and at least once I had to hit the brake myself to stop the car clipping the one beside it, so the technology is far from perfect anyway.