I think more of it has to do with appearance than any sort of profundity everyone seems so fond of blaming. Japan is a HIGHLY image-conscious society, for better or worse, and stuffing your face while on the move isn't a good look, anywhere.
In regard to Japan having this religious respect for food, I don't think anyone who's been to a proper nomikai at an izakaya can say "japan doesn't waste food" with a straight face. I've seen heaps of great stuff NOT taken home in doggy bags because that would be "kakkowarui." And if it weren't the case, leaving the last bite of food for sake of looking like a greedy jerk wouldn't be so commonplace. People would be a lot more "mottainai."
Also, this stuff about japan being this "ultra-clean" society where they don't spill food on the street is rose-tinted idealism. A walk through central Shibuya on a early Sunday morning would not only reveal tons of wasted food discarded by local establishments, but also heaps of litter. Not to mention countless pools of vomit EVERYWHERE, planted for you by salaryman who couldn't hold their liquor the night before.
This isn't to say Japan isn't a deeply beautiful place with profound respect for all things, but I think a lot more boils down to trivialities than anything zen.
Don't limit your perception of japan to what you see in movies, TV, and your favorite new age bookstore. Sometimes a rice ball is just a rice ball. After all, "konbinis" don't exist because everyone believes in the sanctity of corn chips and "American dogs."
Of course, all this aside, despite the taboo against walk-munching, DRINKING (alcohol) in public (on trains, street, etc.) is fairly casual, particularly for men over 40. What about that, then?