As the signs says: No Entry. No need to make up reasons why this should be obeyed.
Some lead to the section ranger's house and apart from privacy issues, access is required 24 hours a day so they can get to emergencies. Most of the roads are single track so imagine having to negotiate a parked vehicle when you need to get somewhere quickly, say to a report of a fire or poaching incident.
Access roads to the section ranger posts are signposted, and I don't see how you could block such a road in the extremely unlikely event of an emergency, unless you are driving a coal mine dump truck.
Most of this roads are fragile and require to be rehabilitated everytime a vehicle drives on them.
Most of these roads require no such thing, any road that needs to be rehabilitated after the passage of a single vehicle cannot be called a road. Indeed, a lot of these roads are fine, and can be negotiated for their entire length in an ordinary vehicle. I know, I've driven some long ones in a 4x2 (with permission) and others in a game viewing vehicle. But sure, you also get "roads" that are two-tracks at best.
No Entry = authorized vehicles only.