Specific example:
Coinmint in upstate new york is located in an old Alcoa aluminum smelter plant that has long shut down. It gets power from Moses Saunders dam a few miles away. The factory was originally placed near the dam so that it could provide massive amounts of power to keep the pot lines hot 24/7.
The whole place is super remote. I've seen the costs for carrying hundreds of MW just a few miles and they are massive at this scale. You can't just economically build more lines to send the power elsewhere.
Short of firing up the superfund site again and smelting aluminum, a bitcoin mine is literally the most profitable thing you could use this power for. The dam needs maintenance and a constant draw of power to keep it operational, and this pays for it.
Since the power lines are direct from the dam to the plant and the contracts were all drawn up decades ago, this has zero impact on the local community prices. Never mind that the local community doesn't draw anywhere near enough power compared with what the dam produces.