The original article was really quite informative about this. Please have a look:
When drivers accept a call, furthermore, they need to
interface with the app. The suit goes on to note that
under California law, it is illegal to use a “wireless
telephone” while driving unless it is specifically
configured to be hands-free — which the app is not. In
essence, the suit argues that Uber was negligent in the
“development, implementation and use of the app” so as
to cause the driver to be distracted and inattentive.
