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.

Overall, it seems from my (possibly limited) view of Japanese culture that they tend to put a lot of emphasis on cohesion, respect of authority and laws, not deviating from what's good for the group as a whole.

Which has its downsides, surely, but in this case is a good thing.

> I need an article to explain why the Japanese clean up for others.

In very broad terms, the culture in Japan is very much "I take care of you, and you take care of me". Sure, it doesn't work the same way abroad, but by the time a Japanese person travels abroad it's already ingrained at a subconscious level.

Source: lived in Japan for 3 years, have a Japan-educated grandmother.

There is also the possibility that the venue itself, a large area filled with people, visiting a foreign country, would cause the to feel such an obligation as "guests".