you answer your own perplexity by yourself in the same sentence; they do it for fun. I know this because I was a shithead vandal teenager, and I did it for two reasons , 1) fun at the moment and 2) to impress my peers and sex partners.
I think the frustrations of youth are undersold here, and I think the expectation of "You're born, now conform to being a cog in this machine that you had nothing to do with or necessarily any interest in." is also somewhat fundamentally flawed, even if the kids have nice bathrooms due to their own efforts to shit in.
a society that repairs itself and allows a certain level of vandalism is better, I think. It allows for the rebellious youth to get through these psychological phases without much possibility of a misstep that allows them to throw themselves into incarceration for life, it offers leeway for personal growth, and it doesn't pin them into programs that they hate and wouldn't integrate into, anyway.
Tell me, how does the whole 'kids take care of their own bathrooms' thing work out for bullies?
answer: they love it. They don't have to work. Asia hasn't solved the bullying thing yet even with their perceived perfect unitary social one-ness.