Should the power company be able to offer you a discounted rate if you host equipment that offsets their cost (for instance: a powerwall)[1]?
I think that the answer to that question is yes.
Well okay, then should your ISP be able to offer you a discounted rate to use services that offset their cost (youtube, which has an cache near you, vs vimeo, which doesn't)? If the power company can do this, then why can't the ISP?
Again, I don't like this, but I can't think of a consistent explanation for why they shouldn't be allowed to dynamically charge you based on their cost.
[1]: I don't know enough about the electrical grid to know if a powerwall could actually be used in a way that offsets the cost to a power utility or not. I don't think that the correctness of this statement matters to the example. Substitute "a powerwall" for "equipment that saves the power company money".