A few reasons. First of all, note that the goal is that for the majority, this is actually a decrease in cost, not an increase. Matching the pollution credit cost would be the right thing if there were no rebate. This is a penalty on above-average use. And it's not just the carbon, it's many other things. Finally, if everybody had to pay to offset the cost of their burning, the credits would cost a great deal more than $20 per tonne, I think. As total output goes down it costs more and more (in theory) to squeeze more out of other locations.