Isn't the 80/20 split done after the power is paid for? That way it is the profit you are splitting. It doesn't make sense otherwise.
Sure your way makes economic sense for the person mining but I think it would become an administrative nightmare for AM to figure out the energy costs for each miner. If AM uses all 60 PH worth of chips - that's 4000 x 15TH franchises.
That is just a problem to solve that would be easier than solving the problem of no one wanting to run a franchise and pay for the power out of their share and no profit. They could pay a flat rate of btc converted at market price per Kwh and potential franchisees could decide if they want to participate or not at those power rates. People with lower power rates would profit slightly more than others.