Now I get it and here I thought it was just a slosh baffle,
Foam in a fuel cell prevents a concentration of the fuel vapor plus air mixture, which significantly reduces the probability that the fuel cell will explode in case of a fire. Reducing the amount of foam in a fuel cell, either by not putting it in to the eight-inch mandated height, or by carving out hollows in the interior of the foam, creates a very highly flammable pocket of fuel vapor and a major safety hazard.
While manipulating the foam might give you an advantage in terms of being able to fit more fuel into the tank, it creates a major, major safety hazard by making the fuel cell more likely to explode in case of an accident. While people talk about messing with the fuel being a major no-no, the big thing here is really the safety aspects.
