This seems to create a potential catch-22. If people are throwing things at pedestrians from the windows/buildings above and the response is to construct fences, nets, scaffolding etc., to protect the pedestrians from the falling objects, this can end up increasing the risk of objects being thrown into the nets/fencing, because now you have created the perception that there is no risk of actually hitting and/or injuring the pedestrians.
So now even more people will throw rubbish out their windows without any intention to hit or even any care or regard for hitting pedestrians, reasoning that the nets/fencing will catch anything they toss out the window anyway. Prior to the nets, only people with actual malice towards the pedestrians would throw things down at them, with actual intent to cause injury, or at a minimum a malicious disregard for the risk of hitting the pedestrians.
I guess just one point of my observation is that the real remedy should have been to criminalize and punish the people throwing things down at the pedestrians or tossing rubbish out of their windows into the street. The whole notion of building nets/fencing instead of punishing the people throwing things out of the windows is insane, and counterproductive, but it appears that there is no power/ability to punish the conduct and/or no desire to by the authorities who would ordinarily do so.