Earlier quoted context omitted.
Why umbrellas specifically? I read the article and I enjoyed it, but the question of why umbrellas are not treated the same in terms of being returned was mentioned but not answered. Near the end of the article, there is this sentence > “Communal property is virtually unheard of except for that many people seem to regard umbrellas as up for grabs if not properly secured.”
You can buy them everywhere for 300-1000 yen, those models are the ones that everyone uses so it isn't much of a sacrifice for everyone to buy another umbrella a few times a year.
So it's not even necessarily a concious system, just the end result of easily being confused between umbrellas.