I think he's saying that if it weren't handled as a political statement, there's no realistic defense against trash being brought into the park. It's the responsibility of the maintainers to notice - not people to prevent the natural flow of life from happening.

People leave cool stuff places. It happens. I mean how many times have cell phones been left somewhere on accident say, which if it was on purpose doesn't matter either -- OH NO, PARK BENCH SSL CERT MITM ATTACKED, PURGE YOUR LUNCHBOXES

No, it's an object left/put there. So unput it there. That's what parks & recreation does all the rest of the time. Now it's a rad piece of art, not a cell phone, not an umbrella, etc.