Permissive bridge.   
Lea Marshes

Although the bridge (next to the closed marina footbridge) is usable and appears to be very safe, right next to it there is an intimidating sign. This is what it says

Permissive Path

This bridge and the way leading to and from it are the Private Property of the Lee Valley Regional Park Authority and is not a Public Right of Way.

Use by the Public is permitted at their own risk and the Authority accepts no liability for any incidents relating to its use.

The right to close this bridge and road at any time without prior notice is reserved.

What does it mean? How can a public authority have private property? What would public property be other than that managed and run by public authorities like Lea Valley Park. Extremely odd!
When we had the permissive society it wasn't like that! Has somebody changed the meaning of the word permissive?