If you've noticed anything from these slides it's that people love to do two things on the subway. They love to sleep and they love to hang. I can understand why you'd do both. We've already covered sleep. Sometimes you just have to sleep on a long commute. As far as hanging goes, subway trains are built like playground. They have bars to hang from all over the place. It's kind of hard not to want to do it (if you're athletic enough to do it).
If I were one of those guys behind him I'd grab his wallet while he was hanging and run off. It's not like he can get down from there quickly without falling. That's what he gets for showing off in public like that. Everyone could use a little bit of humbling every now and then.