I’d say their example is just an oversimplification to keep it understandable. Ultimately fuel based energy has a lot of the same concerns. That natural gas facility costs money to keep viable even if, hypothetically, zero fuel were being burned in some given week. The power lines need repairs, maintenance, upgrades, and expansion over the potential capacity, not actual usage. You have fixed costs alongside the marginal costs. The marginal costs certainly make sense to map directly to usage based rate, but fixed costs are significantly covered by those usage rates as well rather than bumping up the “basic charge” sort of line item on a power bill.