A reason I suspect (though, truly it is only a guess) is as a way to force people to spend money. Something akin to "either buy something or leave" to try and capture just that tiny bit of additional revenue.

"Buy something or leave" has become a kind of sinister, unwritten, yet overarching principle organizing and governing everyone's lives. Participation in the work-earn-spend economy has become less and less optional with each passing day.

I'm not sure that's actually true in a typical park. It may very well be true for a bunch of noisy kids in a shopping mall which exists and is paid for for the purpose of people, well, shopping.

have you looked at satellite maps of the US? there are plenty of places that are concrete jungles with little green space.