[lovemenot]

Fans of any flavour enjoy being subsumed by the crowd and I can imagine this behaviour was a way for those Japanese fans to prolong their feeling of group solidarity after the match.

I'd like to have witnessed the propagation of the clean-up-the-stadium meme as it happened. My guess is that for most of the fans, they'd have been initially ambivalent; the niceties of this situation being unfamiliar. One fan decided it was the right thing to clean-up and their neighbours then propagated the activity throughout the population without giving it too much thought. Once the cleaning was in motion, each fan would have to define their range of cleaning for themselves. Since the area is not specified, they'd err on the conservative side, cleaning a maximal range for any of their possible ejecta. This would reflect a preoccupation on quality over considerations such as opportunity cost or fairness.