From the Airbus performance "Grips" Manual:
"Bending moments, which apply at the wing roots, are maximum when the quantity of fuel in the wings is minimum. During flight, the quantity of fuel located in the wings, Mwf, decreases. As a consequence, it is necessary to limit the weight when there is no fuel in the tanks. This limit value is called Maximum Zero Fuel Weight."
It's a limitation and should not therefore be exceeded.