As Dave Noble has explained its a clear part of the rental conditions with Hertz in Australia.

The issue why its policed heavily now in Brisbane is that the closest petrol station is poorly signposted and hence many people in a rush are simply not refuelling the car when it looks close to full. Previously you would drive past it on your way to either terminal now you need to take the international terminal exit and follow the signs. This policy simply means that everyone is treated the same and with a hope the cars aren't given out not fuel of fuel.

The $3.80/ltr on the left hand side of the RA does not include the Brisbane Airport location fee of 20%.

If you need any further clarification zero savings feel free to flick me a PM.