From the evidence, the owners of these bikes are either mostly pre-teens, or abnormally short.
I don't read Korean so have no idea what sort of establishment they are parked by, but it must generate a certain amount of bike traffic to have merited the two racks. I do often see grown-ups riding bikes for travel in Koreatown--not just poor folks either, and not just twentysomethings but middle-aged folks. Maybe a salubrious habit they brought over from home.
A habit that is spreading, however slowly, to spoiled and timid "real" Americans--who have a lot to learn.