All true electricity markets experience high prices during peak demand. It's just an inevitable consequence of the fact that some users are willing to pay far more money than others to keep the lights on.

We often don't see this, because many electricity "markets" aren't true markets, with real-time bidding. Instead they're based on long term contracts, with the electricity providers taking on substantial financial risks because they're obligated to provide electricity at any cost during high demand and shortages.

Customers like Riot save these providers a lot of money, because they're willing to stop using electricity when the cost of providing that electricity goes up. If things were arranged differently, this would be handled by real-time supply and demand bidding systems: Riot would simply turn off their machines when the price of electricity for that moment reached an unprofitable level, and turn them back on when electricity was cheaper.

For various technical and economic reasons, true real-time bidding isn't implemented that widely in a lot of markets. So instead we get curtailment to accomplish roughly the same economic goal.