Someone further up got it right - this is what happens when social housing is provided as an offshoot of a development rather than having councils or housing associations given the cash to build it themselves. It is also what happens when local authorities are cut to the bone and don't want to provide playgrounds themselves.
Essentially it is two different developments, built at the same time. The social housing block lease will have been transferred to a housing association who pay for the maintenance of that block and the small social housing playground.
The private developer and those that big leases from it pay for the maintenance of their block and whatever facilities are contained within it, gym, bike storage, parking spaces, concierge, playground etc- including insurance in case a kid is injured in the private playground. So legally it's not different from the car parking spaces or conceierge.
It is also highly doubtful the council or the housing association actually want or ever wanted for tenants to have access to the playground because they don't want to be lumbered with a 20/30/40 year commitment to paying for it when most councils are cutting public playgrounds.
If the HA has to contribute to the playground costs, it either has to cut its profit margin or charge its tenants more rent. This would have been agreed at the time of the development being planned in the same way paying for shared driveway or access road would have been
So I am not trying to say it's not a bad thing - but don't think the council and housing association did not know about it from the start regardless of the artwork they used when consulting on the development. The legal contracts would have covered what was and wasn't included in any shared costs.
A better option would be councils given cash to properly fund and maintain public access parks perhaps by taxing offshore property developers and owners properly