Yes, when I was in Japan, and bought korokke from the store, my friend dragged me upto street level across the street, through a shopping mall and out into a park before she would let me sit down and eat it.
Honestly I dont care about drinking in public areas, its hot, im on the move, why would I want to stop every 50 metres to take a sip of my drink? Here in Korea we buy beer in a combini and sit outside and drink it... anywhere. Its a fairly common sight, amongst all ages.
I agree with eating on the go, just being messier. I have been known to eat on a bus once before though, and I did feel bad (this was in Korea), but not so bad when I saw that the bus driver was smoking while driving.
In New Zealand we have signs at the front and middle of the bus saying 'No Food/No Drinking/No feet on chairs' or they have the little symbols for each. Stil I'd be more inclined to eat on a bus there, than I would in Japan.