I think it would be a good law.
I understand people enjoy eating while walking when they go on a trip or go to the food market. I am one of them though.
But in crowded places, we should stop it to avoid getting our clothes dirty when bumping together (and it getting more make crowd because they would walk slowly) - not sure what you are trying to say here.
2:Do you think it's bad manners to eat while walking? Why? Why not?
It depends on the situation.
When I was in Japan, I thought it was bad manners to eat while walking anywhere outside, including the food market.
But since I started to visit foreign countries I am getting used to eating while walking, like when I go to a festival or food market because it is fun. But if we do, we have to follow the rules, for example, throw your waste away in the garbage.