Messiah, the Captain signs a loadsheet acknowledging the amount of fuel on board - assuming the fuel is loaded exactly as ordered - and pax on board using standard weights. This is an accepted method approved by CASA.
If the aircraft is overfuelled, to remain legal, the Captain has one of two options - get the aeroplane defuelled or burn the excess. The second method is far quicker.
Realistically, 400 kg is going to make no difference to a 400 tonne aeroplane's performance or stucture, but that's not the point, is it?
You cannot seriously advocate that a Captain should accept a situation that is not legal, when there are options available to make it so?
Should an accident occur and the Captain knowlingly accepted an aircraft that he or she believed, to the best of their knowledge, to be overweight, they would be hung out to dry by both the airline and the regulator and rightly so.