[People are pigs. Their eating generates lots of garbage and that leads to RATS - the four legged kind - and that leads to infestation and disease. ]
Why not extend that prohibition to city streets? How about prohibiting it in cars, since motorists have this way of throwing garbage out the window? I don’t have strong feelings on the subject, but it’s a little odd that public transit has this special carve-out. It would be nice if we could exclude people with really strong body odor, though (i.e. bums who haven’t showered in weeks).