Those bargain rates are possible largely because the Basin’s five dams generate around six times what the region can use locally. Much of the surplus is exported, at premium prices, to outside buyers such as Puget Sound Energy, and those outside sales subsidize local rates.
But if the utilities are selling more of their power to local Bitcoin miners, they’re exporting less power at those premium prices, which makes it harder to keep local rates low. Further, because much of the surplus is currently committed in long-term contracts, supplying miners with all the megawatts they want might require the utilities do something virtually unprecedented: buy power on the open market, at prices far above what locals have come to expect.
Solution: pass a law to charge miners export prices. It’s not as though Bitcoin mining helps the local economy, so you either return revenues to pre-mining levels, or drive the leeches out. Either outcome is equivalent.