I get the feeling that they are the type of people who don't wear shoes because they are 'feel more grounded that way and why would you wear shoes do you hate earth that much' type people. Probably funded by parents money. I don't really pay them any mind though as long as they don't smell and don't get in the way.
I just don't like when they are too cheap to get a taxi and bring their massive external frame backpack onto the metro during rush hour.
Saw it the last time I was in Tokyo, rush hour on the Yamanote line people packed in like sardines and this cheap guy is trying to bring his massive backpack on the train.