One of the rules for TCS Euro-Truck racing is that you CANNOT add any additional weight to the car as ballast...
16.12 Minimum weight: NONE. No weights, of ANY KIND, may be put on the truck for ballast.
My understanding of this rule is that a heavier Euro-Truck can in some instances improve handling and/or acceleration. Since you are prohibited from adding ballast, you could use a heavier battery to help balance a car or improve mechanical grip. I could see extra weight helping at facilities with very low grip carpet or places that don't allow traction additives.
There are no rules preventing anyone from running a heavier battery, but there can be a point where any advantage gained by weight will be lost due to having to move/accelerate a heavy car. Some examples might be additional motor heat, additional tire wear or loss in battery performance due to having to accelerate a heavier vehicle
I would suspect that someone at a previous TCS event had an excessive amount of ballast added to their vehicle that was either a perceived or real advantage. While this is a good use of "thinking outside the box", it does goes against what I feel is the "spirit" of this type of racing. Since this class is welcome to all level of racers, you don't want to create a perception for novices that there is a need to spend extra money or time trying to circumvent rules to gain additional performance.
I will emphasize that there is nothing preventing any facility or club from running a Semi-Truck based class using whatever rules make sense for them. I am sure there are some clubs that have opened up this type of class to any touring car chassis, any Tamiya hop-ups, or any electronics which would include faster motors. But for TCS Racing the current set of rules work surprisingly well.