For conventional electrical generation, because disposing of large amounts of power safely is more expensive than dumping it onto the grid.

For wind power, because wasting the power before it hits the grid would result in them not getting their federal green energy subsidy.

The same reason people do not take their own trash to the dump, it is cheaper for electrical plants to outsource the wasting of electricity.

At the scale we're talking about it's not trivial - the sheer amount of energy you're dumping has to go somewhere, probably turning into heat one way or the other.

It would be interesting if this could be linked with hydro, and use -ve price points to pump water into reservoirs that can release power a short notice via hydro within a few minutes of demand. Then refill at -ve price points. Using wind and gravity to make money.