Some 20 dollar signs that said, “ No rangers on duty, enter at your own risk” would have worked just as well and allowed the citizenry to enjoy their weekend.
It's the bureaucrats' park, not yours. That's what we get for allowing a government to run the land entertainment business as an armed monopoly.
If you want the real reason, the courts would hold the parks accountable anyway. There is no such thing as "at your own risk" when a deep pocket can be found.