Riding on the sidewalk isn't a green light to run the cyclist over. They're both at fault.
EDIT
Actually, the driver is at fault for the collision, the rider may be guilty of riding on the sidewalk illegally if his wheels are more than 24 inches (stupid law).
The reason the rider took off is probably because there was no reason to hang around. What should he have stayed for?
I believe that has now changed to an age restriction (under 16) in Toronto. At least it was on the table to change about 2 years ago, IIRC.