Most people clean up after themselves, even in hyper individualist America.

The Japanese clean up after everyone including the shitty few who don't clean up after themselves.

Could be because people growing up in Japan are taught that they're an intrinsic part of any place, event, or group of people that includes their presence. Kids in classrooms in Japan are helping clean up together with everyone else at age 4.

It's kinda the opposite of "responsible for yourself," it's a civic sense that extends to include everyone and everything around you - including things that weren't directly caused by you-as-individal.

In the case of the cherry blossoms, they were planted for the enjoyment of the people, and thus the people who come to enjoy them are a part of that system. The cherry blossom viewing events where thousands of people come to picnic, only is a "thing" because thousands of people come - everyone there is a participant by virtue of attending. Thus they hold part of the responsibility for the outcome of the event and the aftermath.