Tossing a large rock off an overpass is no different than randomly opening fire on traffic. Actually, the rock is probably more dangerous.
If the teen had dropped a gun instead of a rock it would have been the gun’s fault. I say all rocks weighing more than an ounce should be registered and a 30 day waiting period should be required before anyone can pick one up. Seriously, the “up to 20 years” will probably mean far less in reality and far short of what should happen to the perp.