Ok, the driver is still being held responsible, which is correct. Sure the braking system failed but the driver was supposed to be there to correct the situation. This the right decision. If the car was fully autonomous then i would agree with you.
The braking system did not fail, both volvo's stock braking system, and Uber's own system were turn off. Uber's self driving software tried to command a braking maneuver over 1.5 seconds before impact, but did not have the authority to do so, as Uber engineers had turned off the functionality due to annoying (or possibly unsafe) false alarms.