Eating and Drinking are rude?

I found out the other day that most Japanese think it’s rude to walk around when drinking or eating. So that explains why I never see anyone walking (or even on the train) with a cup of coffee in the morning. I wonder what Japanese tourists think when they walk around Boston and see every other person walking around with Dunkin’s or Starbucks’ cups?

I never really thought about eating on a train before, mostly because the T is too jumpy and bumpy to eat anything safely. But I can see how eating on the train in Japan could be considered rude,,, have you smelled some Japanese food before?