There have been people injured and killed by drivers who believe that kind of claptrap. The problem is that drivers take time to adjust from staring at a screen to looking ahead at the road. I've seen some estimates that it takes between twenty and forty seconds for a drivers attention to context switch like that. What happens is that a driver barely looks up when the vehicle in front might start to move and they just carelessly think that they should move forwards as well, even though they haven't spotted a filtering cyclist or road-crossing pedestrian.

The law was brought in specifically to try to save lives - hardly a "silly" reason.