> some preferences can be more expensive to deliver than others

I don't think that a kb of facebook is significantly more costly to deliver than a kb of github.

ISPs bill by kbs consumed or a flat rate according to bandwidth availability. In the second case, even if some consumer groups use more bandwidth, it is not consumer's fault. ISPs sell you some capacity, but they underprovision and are therefore affected by total consumption. Without underprovisioning, costs should be out of the equation.

However I agree with you that this point is hard to argue in court.