Afaik the reason the mass dampers (or J-dampers as some other called it) were banned was the same reason FRIC was outlawed. The problem with these systems is that it is very easy to argue it's point is to improve aerodynamics and thus, it is movable aero.Tommy Cookers wrote: about 10 years ago
summarily banning mass dampers ie Renault (so making me ashamed to be British)
R were doing nothing unusual (eg millions of 2Cvs etc) and broke no rule except one that existed only in the CW imagination
conveniently, the competition became closer by CW's rule invention
It's a gray area, always has been and always will be, which means that a large part in deciding what is and isn't legal is feedback from the teams and the arguments present. Some issues are easier than others. and the mass dampers were very easy as the sole reason they were used was aerodynamics.
So yeah, while it's definition is very much questionable the decision(albeit executed rather awkwardly) makes it very clear that rules regarding movable aero were broken.