They don't need to pay 100% of the unit cost, but they do need to pay 100% of the ASIC component cost, otherwise you are just buying a more expensive heater for no reason. The problem is, with 20% up time, there is no hope of ever paying back the 100% ASIC cost, even with the electricity cost savings. Therefore, you are indeed just buying a more expensive heater for no reason.
Ahh... you are making the mistake assuming ASICs cost a lot to manufacture once the initial investment is amortized. It is merely a matter of scale.
You do realize ASIC's exists in many cheap 10-20 dollar children's toys, right?
10-20 dollar children's toy ASIC doesn't consume 1200W electricity. If you are going to use them as heaters, you need a ton of ASIC in there. These ASIC will make the heater cost extremely expensive compared to a regular heater.