Hmmm the few people that clean up at Japanese football games don't actually represent everyone. Many leave drinks containers behind or bento boxes that they brought with them, and often you are given bags of goodies (not actually that exciting, I got a branded folder of the team, and a salad dressing one time) which end up being 'forgotten'. Whilst yes some fans do keep it very tidy, as with many things in Japan, there is an idolised view that isn't based on reality.