If it was true
They would also not insure the following trades that transport water in their vehicles
Car valeters
Truck mount carpet cleaning
Graffiti removal vehicles
Plant watering contractors
Or indeed any other contractor that needs to carry water or heavy machines,materials in their vehicles eg. Cement mixers, Grounds maintenance contractors, Brick layers, Paving contractors etc etc. all of these have large very heavy loads
..... imagine a pallat of bricks or blocks sliding forward in an accident.