Fenced or not, this is just the way it is. A playgroung on the grounds of a block of flats is for their and their guests' use only. It can be enforced by law if necessary, though I would think it very rare.
I remember a house in my childhood where the house manager tried to stop the children of our guests from playing with us in the playground, but that was when my reserved and silent Finnish father took a rapid change into a not-so reserved and silent Finn...
Private Keep Out...
Well, normally the traditional older Finnish man (aka jäärä) just grunts his way through life, doing as he pleases, not giving much thought to the illogicalities of/or rules. My great uncle, for example, thought that as he knows how to drive a car, there's no need to go to driving school. And drove for 50 something years without a license.