I think it was Locke who talked about the "Social Contract". The idea being that society robbed man of the opportunity to live as a "feral" by inventing property rights and restricting their right to roam; therefore society owes each member a subsistence at least equivalent to the one nature would have provided.

I'm not sure I agree with Locke, but I at least feel that the parks are meant to exist as a partial means of living up to that contract and should be less strictly patrolled than this one was on that day.