Drivers should be paying attention to their immediate surroundings while stopped. Especially because the signals may change, or they may need to move for emergencies services or to evade a careening car heading their way.
And also to not incite road rage from their fellow drivers because they are sitting stopped at a green light. You could also cause a fender bender because other drivers, who also may be distracted slightly, are expecting the cars to move and then are startled when they look away for a brief second because the car in front of them had to brake for the car stopped at a green light. Some states hand out tickets for being braked at a green light.
and the one time I feel comfortable looking at Waze to scan the route ahead is when I'm stopped, and an annoying ad is in the way. It's utterly obnoxious and given Google's warchest is unnecessary.