Because they are buying the power, which pays for grid upgrades. The grid won’t be improved without demand, and miners provide a flexible, profitable demand for power.
ERCOT’s incentives are a bit off, though. They should be offering power to miners at very low rates when they have excess supply available, then jacking up the rates to miners well beyond the point of profitability when they don’t have it. Ideally, they would convince the miners to install their own solar and wind generation (and maybe pumped storage as well) and pay them more than they would earn mining to backfeed the grid during power shortages.
Paying miners not to use power is just fucking stupid.