The mills know that. Mining companies know that. That's why a lot of them sell their products forward in the market. They contract for a price into the future. No one should have been surprised.
Electricity is a complex beast.
So is the grid through which it runs. It can't be saved for a rainy day. Generated electricity must be used and used quickly. It runs toward where it's being used, but without an extensive transmission system, it can't be moved great distances. For instance, an energy company with generators in Texas can't just decide to sell power in Montana and start shipping electrons north.