With a top speed of ~12mph, the scooters are way too slow for the road. It's much safer on the sidewalks. The sidewalks in downtown San Jose are also quite wide, so there's plenty of room for pedestrians and scooters to share.
If an olympic athlete who runs at 12mph did his daily run on a sidewalk, people would be really, really angry. 12mph is fast. A police officer would stop him.